Eplain
******

This manual documents the Eplain macros, version 3.14, April 2024.
Eplain provides functionality for plain TeX that is intended to be
useful regardless of how your document is actually formatted.

   Most of this manual is in the public domain, like most of the Eplain
code.  It was originally written by Karl Berry, starting in 1989.
Steven Smith wrote the documentation for the commutative diagram macros;
this chapter is under the GNU General Public License.  Adam Lewenberg
has made additions and corrections.  Oleg Katsitadze wrote the section
on LaTeX packages and the chapter on hyperlinks, and updates throughout.

   The Eplain home page is <https://tug.org/eplain>.

1 Introduction
**************

The “Eplain” macro package expands on and extends the definitions in
plain TeX.  Its home on the web is <https://tug.org/eplain>.

   This manual describes the definitions that you, as either an author
or a macro writer, might like to use.  It doesn't discuss the
implementation; see comments in the source code (‘xeplain.tex’) for
that.

   Eplain is not intended to provide typesetting capabilities, as does
LaTeX (originally written by Leslie Lamport) and Texinfo (originally
written by Richard Stallman).  Instead, it provides definitions that are
intended to be useful regardless of the high-level commands that you use
when you actually prepare your manuscript.

   For example, Eplain does not have a command ‘\section’ to format
section headings in an "appropriate" way, such as LaTeX's ‘\section’.
The philosophy of Eplain is that some people will always need or want to
go beyond the macro designer's idea of "appropriate".  Such canned
macros are fine--as long as you are willing to accept the resulting
output.  If you don't like the results, or if you are trying to match a
different format, you have to put in extra work to override the
defaults.

   On the other hand, almost everyone would like capabilities such as
cross-referencing by labels, so that you don't have to put actual page
numbers in the manuscript.  The author of Eplain is not aware of any
generally available macro packages that (1) do not force their
typographic style on an author, and yet (2) provide such capabilities.

   Besides such generic macros as cross-referencing, Eplain contains
another set of definitions: ones that change the conventions of plain
TeX's output.  For example, math displays in TeX are, by default,
centered.  If you want your displays to come out left-justified, you
have to plow through ‘The TeXbook’ to find some way to do it, and then
adapt the code to your own needs.  Eplain tries to take care of the
messy details of such things, while still leaving the detailed
appearance of the output up to you.

   Finally, numerous definitions turned out to be useful as Eplain was
developed.  They are also documented in this manual, on the chance that
people writing other macros will be able to use them.

   You can send bug reports or suggestions to <tex-eplain@tug.org>.  The
current version number of Eplain is defined as the macro ‘\fmtversion’
at the end of the source file ‘eplain.tex’.  When corresponding, please
refer to it.

   To subscribe to this mailing list (anyone can subscribe, and archives
are public), visit <https://lists.tug.org/tex-eplain>.

   David Walden reported his experience with Eplain as a new user.  The
article is available online at
<https://tug.org/pracjourn/2005-4/walden>.  An introductory article
(written for TUGboat) is also available online at
<https://tug.org/eplain/misc/tb84katsi.pdf>.

   Finally, here are two alternatives to Eplain:

  1. If you just want to load the LaTeX color or graphics package, the
     LaTeX team's ‘miniltx.tex’ suffices:

          \input miniltx.tex
          \input graphicx.tex

  2. If you're interested in a fully-fledged format that shares much of
     Eplain's (and plain.tex's) philosophy, check out OpTeX:
     <https://ctan.org/pkg/optex>.

2 Installation
**************

Your TeX installation should already contain a version of Eplain
(‘eplain.tex’) in its main ‘texmf’ tree; with TeX Live, it's in
‘.../texmf-dist/tex/eplain/eplain.tex’.

   Generally, you can just use that version and there is no need to do
anything else.  But if you want to use a newer version of Eplain for a
given document, you can put the new ‘eplain.tex’ in the document's
directory, and it will be found.

   If you want to install a new ‘eplain.tex’ in some other place, set an
environment variable (commonly, ‘TEXINPUTS’) to tell TeX how to find it.

   Distributions should also create and maintain the ‘eplain.fmt’ file
file.  But if you want to do it yourself, you can:

     $ touch eplain.aux
     $ initex
     This is TeX, ...
     **&plain eplain
     (eplain.tex)
     *\dump
     ... MESSAGES ...

You must make sure that ‘eplain.aux’ exists _before_ you run ‘initex’;
otherwise, warning messages about undefined labels will never be issued.

   You then have to install the resulting ‘eplain.fmt’ in your local
‘texmf’ tree or set an environment variable (‘TEXFORMATS’ to tell TeX
how to find it.  You'll need to consult your distribution's
documentation about that.

3 Invoking Eplain
*****************

The simplest way to use Eplain is simply to put:

     \input eplain

at the beginning of your (plain TeX) input file.  The macro file is
small enough that reading it does not take long.

   However, if a format (‘.fmt’) file has been created for Eplain (see
the previous section), you can eliminate even the time spent reading the
macro source file.  You do this by responding ‘&eplain’ to TeX's ‘**’
prompt.  For example:

     $ tex
     This is TeX, ...
     **&eplain myfile

   In TeX Live and other distributions, a command-line executable named
‘eplain’ is provided which reads the ‘eplain.fmt’ file automatically.

   If you write something which you will be distributing to others, you
won't know if the Eplain format will be loaded already.  If it is, then
doing ‘\input eplain’ will waste time; if it isn't, then you must load
it.  To solve this, Eplain defines the control sequence ‘\eplain’ to be
the letter ‘t’ (a convention borrowed from Lisp; it doesn't matter what
the definition is, only that the definition exists).  Therefore, you can
do the following:

     \ifx\eplain\undefined \input eplain \fi

where ‘\undefined’ must never acquire a definition.

   Eplain consists of several source files:
‘xeplain.tex’
     most of the macros;

‘arrow.tex’
     commutative diagram macros (see Arrow theoretic diagrams),
     written by Steven Smith;

‘btxmac.tex’
     bibliography-related macros (see Citations);

‘iftex.sty’
     sets the switch ‘\ifpdf’ (among other things), which can be used to
     detect direct PDF output (see Checking for PDF output),
     originally written by Heiko Oberdiek;

‘path.sty’
     macro for allowing line breaks at punctuation characters within
     long pathnames, electronic mail addresses, etc., (see Paths),
     written by Philip Taylor;

‘texnames.sty’
     abbreviations for various TeX-related names (see Logos), edited
     by Nelson Beebe.

The file ‘eplain.tex’ is all of these files merged together, with
comments removed.  The original sources can be found in Eplain source
zip archive in your TeX distribution, on CTAN or on Eplain's home page
at <https://tug.org/eplain>.

   All of these files except ‘xeplain.tex’ can be input individually, if
all you want are the definitions in that file.

   Also, since the bibliography macros are fairly extensive, you might
not want to load them, to conserve TeX's memory.  Therefore, if the
control sequence ‘\nobibtex’ is defined, then the bibliography
definitions are skipped.  You must set ‘\nobibtex’ before ‘eplain.tex’
is read, naturally.  For example, you could start your input file like
this:

     \let\nobibtex = t
     \input eplain

By default, ‘\nobibtex’ is not defined, and so the bibliography
definitions _are_ made.

   Likewise, define ‘\noarrow’ if you don't want to include the
commutative diagram macros from ‘arrow.tex’, perhaps because you already
have conflicting ones.

   If you don't want to read or write an ‘aux’ file at all, for any kind
of cross-referencing, define ‘\noauxfile’ before reading ‘eplain.tex’.
This also turns off all warnings about undefined labels.

   Eplain conflicts with AMSTeX (to be precise, with ‘amsppt.sty’): the
macros ‘\cite’ and ‘\ref’ are defined by both.

   If you want to use AMSTeX's ‘\cite’, the solution is to define
‘\nobibtex’ before reading Eplain, as described above.

   If you have ‘amsppt.sty’ loaded and use ‘\ref’, Eplain writes a
warning on your terminal.  If you want to use the AMSTeX ‘\ref’, do
‘\let\ref = \amsref’ after reading Eplain.  To avoid the warning, do
‘\let\ref = \eplainref’ after reading Eplain and before using ‘\ref’.

   Sometimes you may need to run TeX more then once on your ‘.tex’ file
in order to produce and typeset indexes, resolve undefined
cross-references and/or citations.  The shell script ‘texi2dvi’ from the
Texinfo documentation system (see
<https://www.gnu.org/software/texinfo>) can automate this process: it
runs BibTeX, MakeIndex and TeX as many times as needed to complete the
compilation process.  You will need to set the ‘LATEX’ environment
variable to ‘tex’.  For example, in a Bourne-compatible shell, the
following command will do all the work:

     $ LATEX=tex texi2dvi file.tex

Despite the name, ‘texi2dvi’ can also produce ‘.pdf’ output (among other
formats); set ‘LATEX=pdftex’ for this.  See the output from ‘texi2dvi
--help’ for a full list of options.

4 User definitions
******************

This chapter describes definitions that are meant to be used directly in
a document.  When appropriate, ways to change the default formatting are
described in subsections.

4.1 Diagnostics
===============

Plain TeX provides the ‘\tracingall’ command, to turn on the maximum
amount of tracing possible in TeX.  The (usually voluminous) output from
‘\tracingall’ goes both on the terminal and into the transcript file.
It is sometimes easier to have the output go only to the transcript
file, so you can peruse it at your leisure and not obscure other output
to the terminal.  So, Eplain provides the command ‘\loggingall’.  (For
some reason, this command is available in Metafont, but not in TeX.)

   It is also sometimes useful to see the complete contents of boxes.
‘\tracingboxes’ does this.  (It doesn't affect whether or not the
contents are shown on the terminal.)

   You can turn off all tracing with ‘\tracingoff’.

   You can also turn logging on and off globally, so you don't have to
worry about whether or not you're inside a group at the time of command.
These variants are named ‘\gloggingall’ and ‘\gtracingall’.

   Finally, if you write your own help messages (see ‘\newhelp’ in ‘The
TeXbook’), you want a convenient way to break lines in them.  This is
what TeX's ‘\newlinechar’ parameter is for; however, plain TeX doesn't
set ‘\newlinechar’.  Therefore, Eplain defines it to be the character
‘^^J’.

   For example, one of Eplain's own error messages is defined as
follows:

     \newhelp\envhelp{Perhaps you forgot to end the previous^^J%
        environment? I'm finishing off the current group,^^J%
        hoping that will fix it.}%

4.2 Rules
=========

The default dimensions of rules are defined in chapter 21 of the ‘The
TeXbook’.  To sum up what is given there, the "thickness" of rules is
0.4pt by default.  Eplain defines three parameters that let you change
this dimension: ‘\hruledefaultheight’, ‘\hruledefaultdepth’, and
‘\vruledefaultwidth’.  By default, they are defined as ‘The TeXbook’
describes.

   But it would be wrong to redefine ‘\hrule’ and ‘\vrule’.  For one
thing, some macros in plain TeX depend on the default dimensions being
used; for another, rules are used quite heavily, and the performance
impact of making it a macro can be noticeable.  Therefore, to take
advantage of the default rule parameters, you must use ‘\ehrule’ and
‘\evrule’.

4.3 Citations and bibliographies
================================

Bibliographies are part of almost every technical document.  To handle
them conveniently, you need two things: a program to do the tedious
formatting, and a way to cite references by labels, rather than by
numbers.  The BibTeX program, written by Oren Patashnik, takes care of
the first item; the citation commands in LaTeX, written to be used with
BibTeX, take care of the second.  Therefore, Eplain adopts the use of
BibTeX, and virtually the same interface as LaTeX.

   The general idea is that you put citation commands in the text of
your document, and commands saying where the bibliography data is.  When
you run TeX, these commands produce output on the file with the same
root name as your document (by default) and the extension ‘.aux’.
BibTeX reads this file.  You should put the bibliography data in a file
or files with the extension ‘.bib’.  BibTeX writes out a file with the
same root name as your document and extension ‘.bbl’.  Eplain reads this
file the next time you run your document through TeX.  (It takes
multiple passes to get everything straight, because usually after seeing
your bibliography typeset, you want to make changes in the ‘.bib’ file,
which means you have to run BibTeX again, which means you have to run
TeX again...) An annotated example of the whole process is given below.

   If your document has more than one bibliography--for example, if it
is a collection of papers--you can tell Eplain to use a different root
name for the ‘.bbl’ file by defining the control sequence
‘\bblfilebasename’.  The default definition is simply ‘\jobname’.

   On the other hand, if your document's bibliography is very simple,
you may prefer to create the ‘.bbl’ file yourself, by hand, instead of
using BibTeX.  An annotated example of this approach is also given
below.

   See the document ‘BibTeXing’ (whose text is in the file ‘btxdoc.tex’,
which should be in the Eplain distribution you got) for information on
how to write your .bib files.  Both the BibTeX and the Eplain
distributions contain several examples, also.

   The ‘\cite’ command produces a citation in the text of your document.
The exact printed form the citation will take is under your control
(see Formatting citations).  ‘\cite’ takes one required argument, a
comma-separated list of cross-reference labels (see
Cross-references, for exactly what characters are allowed in such
labels).  Warning: spaces in this list are taken as part of the
following label name, which is probably not what you expect.  The
‘\cite’ command also produces a command in the .aux file that tells
BibTeX to retrieve the given reference(s) from the .bib file.  ‘\cite’
also takes one optional argument, which you specify within square
brackets, as in LaTeX.  This text is simply typeset after the citations.
(See the example below.)

   Eplain can create hypertext links for citations pointing to the
relevant bibliography entries (see Citation hyperlinks).

   Another command, ‘\nocite’, puts the given reference(s) into the
bibliography, but produces nothing in the text.

   The ‘\bibliography’ command is next.  It serves two purposes:
producing the typeset bibliography, and telling BibTeX the root names of
the .bib files.  Therefore, the argument to ‘\bibliography’ is a comma
separated list of the .bib files (without the ‘.bib’).  Again, spaces in
this list are significant.

   You tell BibTeX the particular style in which you want your
bibliography typeset with one more command: ‘\bibliographystyle’.  The
argument to this is a single filename STYLE, which tells BibTeX to look
for a file STYLE.bst.

   Numerous styles have been defined by now; see
<https://www.ctan.org/topic/bibtex-sty>.  One particular case: the
‘apalike’ semi-standard style requires ‘\input apalike.tex’ to function
properly (else text is overwritten); other APA-like ("humanities")
styles surely also require this or similar changes.

   See the document ‘Designing BibTeX styles’ (whose text is in the
‘btxhak.tex’) for information on how to write your own styles.

   Eplain automatically reads the citations from the .aux file when your
job starts.

   If you don't want to see the messages about undefined citations, you
can say ‘\xrefwarningfalse’ before making any citations.  Eplain
automatically does this if the .aux file does not exist.  You can
restore the default by saying ‘\xrefwarningtrue’.

   Here is a TeX input file that illustrates the various commands.

     \input eplain                    % Reads the .aux file.
     Two citations to Knuthian works:
       \cite[note]{surreal,concrete-math}.
     \beginsection{References.}\par   % Title for the bibliography.
     \bibliography{knuth}             % Use knuth.bib for the labels.
     \bibliographystyle{plain}        % Number the references.
     \end                             % End of the document.

   If we suppose that this file was named ‘citex.tex’ and that the
bibliography data is in ‘knuth.bib’ (as the above ‘\bibliography’
command says), the following commands do what's required.  (‘$ ’
represents the shell prompt.)

     $ tex citex     (produces undefined citation messages)
     $ bibtex citex  (read knuth.bib and citex.aux, write citex.bbl)
     $ tex citex     (read citex.bbl, still have undefined citations)
     $ tex citex     (one more time, to resolve the references)

The ‘texi2dvi’ program can help you automate this process (see
Invoking Eplain).

   For simple documents you might choose to write the ‘.bbl’ file
yourself, instead of running BibTeX.  For this scenario, the following
commands should suffice:

     $ tex citex     (read citex.bbl, produces undefined citation messages)
     $ tex citex     (one more time, to resolve the references)

   The output looks something like (because we used the plain
bibliography style):

     Two citations to Knuthian works: [2,1 note].

     References

     [1] Ronald L. Graham, Donald E. Knuth, and Oren Patashnik. Concrete
     Mathematics. Addison-Wesley, Reading, Massachusetts, 1989.

     [2] Donald E. Knuth. Surreal Numbers. Addison-Wesley, Reading,
     Massachusetts, 1974.

   See the BibTeX documentation for information on how to write the
bibliography databases, and the bibliography styles that are available.
(If you want your references printed with names, as in [Knu74], instead
of numbered, the bibliography style is ‘alpha’.)

4.3.1 Formatting citations
--------------------------

You may wish to change Eplain's formatting of citations; i.e., the
result of your ‘\cite’ commands. By default, the citation labels are
printed one after another, separated by commas and enclosed in brackets,
using the main text font. Some formats require other styles, such as
superscripted labels. You can accommodate such formats by redefining the
following macros.

‘\printcitestart’
‘\printcitefinish’
     Eplain expands these macros at the beginning and end of the list of
     citations for each ‘\cite’ command. By default, they produce a ‘[’
     and ‘]’, respectively.

‘\printbetweencitations’
     If a ‘\cite’ command has multiple citations, as in
     ‘\cite{acp,texbook}’, Eplain expands this macro in between each
     pair of citations. By default, it produces a comma followed by a
     space.

‘\printcitenote’
     This macro takes one argument, which is the optional note to the
     ‘\cite’ command. If the ‘\cite’ command had no note, this macro
     isn't used. Otherwise, it should print the note. By default, the
     note is preceded with a comma and a space.

   Here is an example, showing you could produce citations as
superscripted labels, with the optional notes in parentheses.

     \def\printcitestart{\unskip $^\bgroup}
     \def\printbetweencitations{,}
     \def\printcitefinish{\egroup$}
     \def\printcitenote#1{\hbox{\sevenrm\space (#1)}}

4.3.2 Formatting bibliographies
-------------------------------

You may wish to change Eplain's formatting of the bibliography,
especially with respect to the fonts that are used. Therefore, Eplain
provides the following control sequences:

‘\biblabelwidth’
     This control sequence represents a ‘\dimen’ register, and its value
     is the width of the widest label in the bibliography. Although it
     is unlikely you will ever want to redefine it, you might want to
     use it if you redefine ‘\biblabelprint’, below.

‘\biblabelprint’
     This macro takes one argument, the label to print. By default, the
     label is put in a box of width ‘\biblabelwidth’, and is followed by
     an enspace. When you want to change the spacing around the labels,
     this is the right macro to redefine.

‘\biblabelcontents’
     This macro also takes one argument, the label to print. By default,
     the label is printed using the font ‘\bblrm’ (below), and enclosed
     in brackets. When you want to change the appearance of the label,
     but not the spacing around it, this is the right macro to redefine.

‘\biblabelprecontents’
‘\biblabelpostcontents’
     Macros expanded before and after ‘\biblabelcontents’, respectively.
     For example, to get left-justified numeric labels (they are
     right-justified by default):

          \def\biblabelprecontents{\relax}
          \def\biblabelpostcontents{\hss}

‘\bblrm’
     The default font used for printing the bibliography.

‘\bblem’
     The font used for printing the titles and other "emphasized"
     material.

‘\bblemph’
     Typesets its argument using ‘\bblem’, then inserts an italic
     correction.

‘\bblsc’
     In some styles, authors' names are printed in a caps-and-small-caps
     font. In those cases, this font is used.

‘\bblnewblock’
     This is invoked between each of the parts of a bibliography entry.
     The default is to leave some extra space between the parts; you
     could redefine it to start each part on a new line (for example). A
     part is simply a main element of the entry; for example, the author
     is a part. (It was LaTeX that introduced the (misleading, as far as
     I am concerned) term 'block' for this.)

‘\biblabelextraspace’
     Bibliography entries are typeset with a hanging indentation of
     ‘\biblabelwidth’ plus this. The default is ‘.5em’, where the em
     width is taken from the ‘\bblrm’ font. If you want to change this,
     you should do it inside ‘\bblhook’.

‘\bblhook’
     This is expanded before reading the .bbl file. By default, it does
     nothing. You could, for example, define it to set the bibliography
     fonts, or produce the heading for the references. Two spacing
     parameters must be changed inside ‘\bblhook’: ‘\parskip’, which
     produces extra space between the items; and ‘\biblabelextraspace’,
     which is described above. (By the way, ‘\hookappend’ won't work
     with ‘\bblhook’, despite the names. Just use ‘\def’.)

   If you are desperate, of course you can also hand-edit the .bbl file
that BibTeX produces to do anything you wish.

4.3.3 Commands from LaTeX
-------------------------

Because of the historical connection between BibTeX and LaTeX, in
practice many bibliography styles and bibliographies use LaTeX commmands
that are not part of bibliography handling, per se.

   To support this, ‘btxmac.tex’ (and thus Eplain) define the following.
In all cases, an existing definition (e.g., from ‘miniltx.tex’, see
Loading LaTeX packages) will not be
overwritten. Here is the list:

‘\newcommand’
‘\renewcommand’
‘\providecommand’
     Defining new commands in various ways. The Eplain versions do not
     support the *-form of these; use ‘miniltx’ for that.

‘\em’
‘\emph’
‘\sc’
‘\textbf’
     Selecting fonts.

‘\mbox’
     A horizontal box.

‘\newblock’
     Starts elements of a bibliography entry.

   For full information about these, see the LaTeX manual and sources.

4.4 Displays
============

By default, TeX centers displayed material. (Displayed material is just
whatever you put between ‘$$’'s--it's not necessarily mathematics.) Many
layouts would be better served if the displayed material was
left-justified. Therefore, Eplain provides the command ‘\leftdisplays’,
which indents displayed material by ‘\parindent’ plus ‘\leftskip’, plus
‘\leftdisplayindent’.

   You can go back to centering displays with ‘\centereddisplays’. (It
is usually poor typography to have both centered and left-justified
displays in a single publication, though.)

   ‘\leftdisplays’ also changes the plain TeX commands that deal with
alignments inside math displays, ‘\displaylines’, ‘\eqalignno’, and
‘\leqalignno’, to produce left-justified text. You can still override
this formatting by inserting ‘\hfill’ glue, as explained in ‘The
TeXbook’.

   Eplain defines ‘\eqnum’ and ‘\eqalignnum’ which can be set up to
produce either left-aligned or right-aligned equation numbers.
‘\lefteqnumbers’ (‘\righteqnumbers’) will define ‘\eqnum’ to expand to
‘\eqno’ (‘\leqno’), and ‘\eqalignnum’ to expand to ‘\eqalignno’
(‘\leqalignno’). Default is ‘\righteqnumbers’ (right-aligned equation
numbers).

4.4.1 Formatting displays
-------------------------

If you want some other kind of formatting, you can write a definition of
your own, analogous to ‘\leftdisplays’. You need only make sure that
‘\leftdisplaysetup’ is called at the beginning of every display
(presumably by invoking it in TeX's ‘\everydisplay’ parameter).

   ‘\leftdisplays’ expands the old value of ‘\everydisplay’ before
calling ‘\leftdisplaysetup’, so that any changes you have made to it
won't be lost. That old token list as available as the value of the
token register ‘\previouseverydisplay’.

4.5 Time of day
===============

TeX provides the day, month, and year as numeric quantities (unless your
TeX implementation is woefully deficient). Eplain provides some control
sequences to make them a little more friendly to humans.

   ‘\monthname’ produces the name of the current month, abbreviated to
three letters.

   ‘\fullmonthname’ produces the name of the current month,
unabbreviated (in English).

   ‘\timestring’ produces the current time, as in '1:14 p.m.'

   ‘\timestamp’ produces the current date and time, as in '23 Apr 64
1:14 p.m.'. (Except the spacing is slightly different.)

   ‘\today’ produces the current date, as in '23 April 1964'.

4.6 Lists
=========

Many documents require lists of items, either numbered or simply
enumerated. Plain TeX defines one macro to help with creating lists,
‘\item’, but that is insufficient in many cases. Therefore, Eplain
provides two pairs of commands:

‘\numberedlist ... \endnumberedlist’
‘\orderedlist ... \endorderedlist’
     These commands (they are synonyms) produce a list with the items
     numbered sequentially, starting from one. A nested ‘\numberedlist’
     labels the items with lowercase letters, starting with 'a'. Another
     nested ‘\numberedlist’ labels the items with roman numerals. Yet
     more deeply nested numbered lists label items with ‘*’.

‘\unorderedlist ... \endunorderedlist’
     This produces a list with the items labelled with small black boxes
     ("square bullets"). A nested ‘\unorderedlist’ labels items with
     em-dashes. Doubly (and deeper) nested unordered lists label items
     with '*'s.

   The two kinds of lists can be nested within each other, as well.

   In both kinds of lists, you begin an item with ‘\li’. An item may
continue for several paragraphs. Each item starts a paragraph.

   You can give ‘\li’ an optional argument, a cross-reference label.
It's defined to be the "marker" for the current item. This is useful if
the list items are numbered. You can produce the value of the label with
‘\xrefn’. See Cross-references.

   Eplain can create hypertext links for the markers produced by
‘\xrefn’ pointing to the relevant list item (see List hyperlinks).

   You can also say ‘\listcompact’ right after ‘\numberedlist’ or
‘\unorderedlist’. The items in the list will then not have any extra
space between them (see Formatting lists). You might want to do this
if the items in this particular list are short.

   Here is an example:

     \numberedlist\listcompact
     \li The first item.
     \li The second item.

     The second paragraph of the second item.
     \endnumberedlist

4.6.1 Formatting lists
----------------------

Several registers define the spacing associated with lists. It is likely
that their default values won't suit your particular layout.

‘\abovelistskipamount, \belowlistskipamount’
     The vertical glue inserted before and after every list,
     respectively.

‘\interitemskipamount’
     The vertical glue inserted before each item except the first.
     ‘\listcompact’ resets this to zero, as mentioned above.

‘\listleftindent, \listrightindent’
     ‘\listrightindent’ is the amount of space by which the list is
     indented on the right; i.e., it is added to ‘\rightskip’.
     ‘\listleftindent’ is the amount of space, _relative to_
     ‘\parindent’, by which the list is indented on the left. Why treat
     the two parameters differently? Because (a) it is more useful to
     make the list indentation depend on the paragraph indentation;
     (b) footnotes aren't formatted right if ‘\parindent’ is reset to
     zero.

   The three vertical glues are inserted by macros, and preceded by
penalties: ‘\abovelistskip’ does ‘\vpenalty\abovelistpenalty’ and then
‘\vskip\abovelistskip’. ‘\belowlistskip’ and ‘\interitemskip’ are
analogous.

   In addition, the macro ‘\listmarkerspace’ is called to separate the
item label from the item text. This is set to ‘\enspace’ by default.

   If you want to change the labels on the items, you can redefine these
macros: ‘\numberedmarker’ or ‘\unorderedmarker’. The following registers
might be useful if you do:

‘\numberedlistdepth, \unorderedlistdepth’
     These keep track of the depth of nesting of the two kinds of lists.

‘\itemnumber, \itemletter’
     These keep track of the number of items that have been seen in the
     current numbered list. They are both integer registers. The
     difference is that ‘\itemnumber’ starts at one, and ‘\itemletter’
     starts at 97, i.e., lowercase 'a'.

   You can also redefine the control sequences that are used internally,
if you want to do something radically different: ‘\beginlist’ is invoked
to begin both kinds of lists; ‘\printitem’ is invoked to print the label
(and space following the label) for each item; and ‘\endlist’ is invoked
to end both kinds of lists.

4.7 Verbatim listing
====================

It is sometimes useful to include a file verbatim in your document; for
example, part of a computer program. The ‘\listing’ command is given one
argument, a filename, and produces the contents of that file in your
document. ‘\listing’ expands ‘\listingfont’ to set the current font. The
default value of ‘\listingfont’ is ‘\tt’.

   You can take arbitrary actions before reading the file by defining
the macro ‘\setuplistinghook’. This is expanded just before the file is
input.

   If you want to have line numbers on the output, you can say
‘\let\setuplistinghook = \linenumberedlisting’. The line numbers are
stored in the count register ‘\lineno’ while the file is being read. You
can redefine the macro ‘\printlistinglineno’ to change how they are
printed.

   Normally, the ‘\listing’ command will add a final empty line at the
end of the output, even if the file does not end in a newline. To
suppress this final line, you can say ‘\let\setuplistinghook =
\nolastlinelisting’. This also works with line numbers (say
‘\def\setuplistinghook{\linenumberedlisting \nolastlinelisting}’), but
only if ‘\printlistinglineno’ consists exclusively of boxes at the top
level (i.e., any ‘\kern’s or glue should be wrapped up in a box).

   You can use the form feed control character (ASCII code 12, typed as
‘CTRL-L’) in the file to force a page break in the output.

   You can produce in-line verbatim text in your document with
‘\verbatim’. End the text with ‘|endverbatim’. If you need a ‘|’ in the
text, double it. If the first character of the verbatim text is a space,
use ‘| ’. (‘| ’ will work elsewhere in the argument, too, but isn't
necessary.)

   For example:

     \verbatim| ||\#%&!|endverbatim

produces ‘ |\#%&!’.

   Line breaks and spaces in the verbatim text are preserved.

   You can change the verbatim escape character from the default ‘|’
with ‘\verbatimescapechar CHAR’; for example, this changes it to ‘@’.

     \verbatimescapechar \@

The backslash is not necessary in some cases, but is in others,
depending on the catcode of the character. The argument to
‘\verbatimescapechar’ is used as ‘\catcode `CHAR’, so the exact rules
follow that for ‘\catcode’.

   To reset the category code of all special characters to 12 ("other"),
‘\verbatim’ uses ‘\uncatcodespecials’ (see Category codes). If you
make additional characters "special", you should extend ‘\dospecials’ to
include those characters, lest they be given special treatment inside
verbatim environments. For example,

     \catcode`\A=\active
     % Try commenting out the following line.
     \expandafter\def\expandafter\dospecials\expandafter{\dospecials\do\A}
     \verbatimA#$%_^|endverbatim

   Because ‘\verbatim’ must change the category code of special
characters, calling inside a macro definition of your own does not work
properly. For example:

     \def\mymacro{\verbatim &#%|endverbatim}% Doesn't work!

To accomplish this, you must change the category codes yourself before
making the macro definition. Perhaps ‘\uncatcodespecials’ will help you
(see Category codes).

4.8 Contents
============

Producing a table of contents that is both useful and aesthetic is one
of the most difficult design problems in any work. Naturally, Eplain
does not pretend to solve the design problem. Collecting the raw data
for a table of contents, however, is much the same across documents.
Eplain uses an auxiliary file with extension ‘.toc’ (and the same root
name as your document) to save the information.

4.8.1 Writing the .toc file
---------------------------

To write an entry for the table of contents, you say
‘\writetocentry{PART}{TEXT}’, where PART is the type of part this entry
is, e.g., ‘chapter’, and TEXT is the text of the title. ‘\writetocentry’
puts an entry into the .toc file that looks like
‘\tocPARTentry{TEXT}{PAGE NUMBER}’ (unless PART is an integer, see
below). The TEXT is written unexpanded.

   A related command, ‘\writenumberedtocentry’, takes one additional
argument, the first token of which is expanded at the point of the
‘\writenumberedtocentry’, but the rest of the argument is not expanded.
The usual application is when the parts of the document are numbered. On
the other hand, the one-level expansion allows you to use the argument
for other things as well (author's names in a proceedings, say), and not
have accents or other control sequences expanded. The downside is that
if you _want_ full expansion of the third argument, you don't get
it--you must expand it yourself, before you call
‘\writenumberedtocentry’.

   For example:

     \writenumberedtocentry{chapter}{A $\sin$ wave}{\the\chapno}
     \writetocentry{section}{A section title}

Supposing ‘\the\chapno’ expanded to ‘3’ and that the ‘\write’'s occurred
on pages eight and nine, respectively, the above writes the following to
the .toc file:

     \tocchapterentry{A $\sin$ wave}{3}{8}
     \tocsectionentry{A section title}{9}

   A variation on ‘\writenumberedtocentry’ is ‘\writenumberedtocline’,
differing only in the order of the parameters it takes and writes for
the ‘\tocPARTentry’ control sequences. To continue the previous example:

     \writenumberedtocline{chapter}{\the\chapno}{A $\sin$ wave}

writes the following to the .toc file:

     \tocchapterentry{3}{A $\sin$ wave}{8}

Such ordering of the parameters allows the ‘\tocPARTentry’ macros to
typeset the text of the entry without actually reading it as an
argument. This is required for entries which need to change character
catcodes, e.g., to produce verbatim text (see Verbatim listing).

   Each of ‘\writetocentry’, ‘\writenumberedtocentry’ and
‘\writenumberedtocline’ processes a numeric PART argument specially. If
you pass PART expanding to an integer, these macros write into the .toc
file an entry that starts with ‘\tocentry{PART}’. Thus, you can define a
single ‘\tocentry’ macro which formats all entries for a table of
contents. To continue the previous examples:

     \writenumberedtocentry{1}{A $\sin$ wave}{\the\chapno}
     \writenumberedtocline{1}{\the\chapno}{A $\sin$ wave}
     \writetocentry{2}{A section title}

writes the following to the .toc file:

     \tocentry{1}{A $\sin$ wave}{3}{8}
     \tocentry{1}{3}{A $\sin$ wave}{8}
     \tocentry{2}{A section title}{9}

4.8.2 Reading the .toc file
---------------------------

You read the .toc file with the command ‘\readtocfile’. Naturally,
whatever ‘\toc... entry’ commands that were written to the file must be
defined when ‘\readtocfile’ is invoked. Eplain has minimal definitions
for ‘\tocchapterentry’, ‘\tocsectionentry’, and ‘\tocsubsectionentry’,
just to prevent undefined control sequence errors in common cases. They
aren't suitable for anything but preliminary proofs.

   Each of ‘\writetocentry’, ‘\writenumberedtocentry’ and
‘\writenumberedtocline’ opens the .toc file for writing, thereby
deleting the information from the previous run. You should therefore
arrange that ‘\readtocfile’ be called before the first call to a
‘\writetoc...’ macro, or after the last call. You can't arbitrarily mix
reading and writing.

   ‘\readtocfile’ does not itself delete the information from the .toc
file, so that you can call it several times, e.g., to create both a
short and normal table of contents. (To produce this particular case,
define ‘\tocsectionentry’ to produce nothing while you are reading .toc
file for a short table of contents (see Macro arguments).)

   On the other hand, if you don't want to rewrite the .toc file at all,
perhaps because you are only running TeX on part of your manuscript, you
can set ‘\rewritetocfilefalse’.

4.8.3 Changing the .toc file's root name
----------------------------------------

By default, the ‘.toc’ file has the root ‘\jobname’. If your document
has more than one contents--for example, if it is a collection of
papers, some of which have their own contents--you can tell Eplain to
use a different root name by defining the control sequence
‘\tocfilebasename’.

   Note that ‘\writetocentry’, ‘\writenumberedtocentry’ and
‘\writenumberedtocline’ will open the contents file for writing only at
the first call, using the value of ‘\tocfilebasename’ at that time.
Changing the value of ‘\tocfilebasename’ afterwards will not affect
which file gets _written_, although it will affect which file gets
_read_ by ‘\readcontentsfile’. In case you need to write several
contents files from a single TeX job, use ‘\definecontentsfile’ (see
Alternative contents files).

4.8.4 Alternative contents files
--------------------------------

In addition to the usual table of contents, you may want to have a list
of figures, list of tables, or other such contents-like list. You can do
this with ‘\definecontentsfile{ABBREV}’. All of the above commands are
actually a special case that Eplain predefines with

     \definecontentsfile{toc}

The ABBREV is used both for the file extension and in the control
sequence names.

4.9 Cross-references
====================

It is often useful to refer the reader to other parts of your document;
but putting literal page, section, equation, or whatever numbers in the
text is certainly a bad thing.

   Eplain therefore provides commands for symbolic cross-references. It
uses an auxiliary file with extension .aux (and the same root name as
your document) to keep track of the information. Therefore, it takes two
passes to get the cross-references right--one to write them out, and one
to read them in. Eplain automatically reads the .aux file at the first
reference; after reading it, Eplain reopens it for writing.

   You can control whether or not Eplain warns you about undefined
labels. See Citations.

   Labels in Eplain's cross-reference commands can use characters of
category code eleven (letter), twelve (other), ten (space), three (math
shift), four (alignment tab), seven (superscript), or eight (subscript).
For example, ‘(a1 $&^_’ is a valid label (assuming the category codes of
plain TeX), but ‘%#\{’ has no valid characters.

   You can also do symbolic cross-references for bibliographic citations
and list items. See Citations, and Lists.

   Eplain can create hypertext links for the cross-references (see
Cross-reference hyperlinks).

4.9.1 Defining generic references
---------------------------------

Eplain provides the command ‘\definexref’ for general cross-references.
It takes three arguments: the name of the label (see section above for
valid label names), the value of the label (which can be anything), and
the "class" of the reference--whether it's a section, or theorem, or
what. For example:

     \definexref{sec-intro}{3.1}{section}

Of course, the label value is usually generated by another macro using
TeX count registers or some such.

   ‘\definexref’ doesn't actually define LABEL; instead, it writes out
the definition to the .aux file, where Eplain will read it on the next
TeX run.

   The CLASS argument is used by the ‘\ref’ and ‘\refs’ commands. See
the next section.

4.9.2 Using generic references
------------------------------

To retrieve the value of the label defined via ‘\definexref’ (see the
previous section), Eplain provides the following macros:

‘\refn{LABEL}’
‘\xrefn{LABEL}’
     ‘\refn’ and ‘\xrefn’ (they are synonyms) produce the bare
     definition of LABEL. If LABEL isn't defined, issue a warning, and
     produce LABEL itself instead, in typewriter. (The warning isn't
     given if ‘\xrefwarningfalse’.)

‘\ref{LABEL}’
     Given the class C for LABEL (see the description of ‘\definexref’
     in the previous section), expand the control sequence ‘\C word’ (if
     it's defined) followed by a tie. Then call ‘\refn’ on LABEL.
     (Example below.)

‘\refs{LABEL}’
     Like ‘\ref’, but append the letter ‘s’ to the ‘\...word’.

   The purpose of the ‘\...word’ macro is to produce the word 'Section'
or 'Figure' or whatever that usually precedes the actual reference
number.

   Here is an example:

     \def\sectionword{Section}
     \definexref{sec-intro}{3.1}{section}
     \definexref{sec-next}{3.2}{section}
     See \refs{sec-intro} and \refn{sec-next} ...

This produces 'See Sections 3.1 and 3.2 ...'

4.10 Page references
====================

Eplain provides two commands for handling references to page numbers,
one for definition and one for use.

‘\xrdef{LABEL}’
     Define LABEL to be the current page number. This produces no
     printed output, and ignores following spaces.

‘\xref{LABEL}’
     Produce the text 'p. PAGENO', which is the usual form for
     cross-references. The PAGENO is actually LABEL's definition; if
     LABEL isn't defined, the text of the label itself is printed. The
     'p. ' prefix is defined by ‘\xrefpageword’. Its default definition
     is ‘p.\thinspace’.

   Eplain can create hypertext links for the page references (see Page reference hyperlinks).

4.11 Equation references
========================

Instead of referring to pages, it's most useful if equation labels refer
to equation numbers. Therefore, Eplain reserves a ‘\count’ register,
‘\eqnumber’, for the current equation number, and increments it at each
numbered equation.

   Here are the commands to define equation labels and then refer to
them:

‘\eqdef{LABEL}’
     This defines LABEL to be the current value of ‘\eqnumber’, and, if
     the current context is not inner, then produces a ‘\eqnum’ command
     (see Displays). (The condition makes it possible to use
     ‘\eqdef’ in an ‘\eqalignno’ construction, for example.) The text of
     the equation number is produced using ‘\eqprint’. See Formatting equation references.

     If LABEL is empty, you still get an equation number (although
     naturally you can't reliably refer to it). This is useful if you
     want to put numbers on all equations in your document, and you
     don't want to think up unique labels.

     To refer to the last equation with the empty label, you use the
     empty label in one of the equation reference macros (see below).
     This can be handy when you want to refer to an equation shortly
     after its definition, say, in the sentence following the displayed
     equation, and do not intend to refer to the equation later. But use
     this trick with extreme caution: if later you change the text and
     insert another empty definition between the original definition and
     the reference, the reference will start to refer to the new
     empty-labeled equation.

‘\eqdefn{LABEL}’
     This is like ‘\eqdef’, except it always omits the ‘\eqnum’ command.
     It can therefore be used in places where ‘\eqdef’ can't; for
     example, in a non-displayed equation. The text of the equation
     number is not produced, so you can also use it in the (admittedly
     unusual) circumstance when you want to define an equation label but
     not print that label.

‘\eqref{LABEL}’
     This produces a formatted reference to LABEL. If LABEL is undefined
     (perhaps because it is a forward reference), it just produces the
     text of the label itself. Otherwise, it calls ‘\eqprint’.

‘\eqrefn{LABEL}’
     This produces the cross-reference text for LABEL. That is, it is
     like ‘\eqref’, except it doesn't call ‘\eqprint’.

   Equation labels can contain the same characters that are valid in
general cross-references.

   Eplain can create hypertext links for the equation references (see
Equation reference hyperlinks).

4.11.1 Formatting equation references
-------------------------------------

Both defining an equation label and referring to it should usually
produce output. This output is produced with the ‘\eqprint’ macro, which
takes one argument, the equation number being defined or referred to. By
default, this just produces ‘(NUMBER)’, where NUMBER is the equation
number. To produce the equation number in a different font, or with
different surrounding symbols, or whatever, you can redefine ‘\eqprint’.
For example, the following definition would print all equation numbers
in italics. (The extra braces define a group, to keep the font change
from affecting surrounding text.)

     \def\eqprint#1{{\it (#1)}}

   In addition to changing the formatting of equation numbers, you might
want to add more structure to the equation number; for example, you
might want to include the chapter number, to get equation numbers like
'(1.2)'. To achieve this, you redefine ‘\eqconstruct’. For example:

     \def\eqconstruct#1{\the\chapternumber.#1}

(If you are keeping the chapter number in a count register named
‘\chapternumber’, naturally.)

   The reason for having both ‘\eqconstruct’ and ‘\eqprint’ may not be
immediately apparent. The difference is that ‘\eqconstruct’ affects the
text that cross-reference label is defined to be, while ‘\eqprint’
affects only what is typeset on the page. The example just below might
help.

   Usually, you want equation labels to refer to equation numbers. But
sometimes you might want a more complicated text. For example, you might
have an equation '(1)', and then have a variation several pages later
which you want to refer to as '(1*)'.

   Therefore, Eplain allows you to give an optional argument (i.e.,
arbitrary text in square brackets) before the cross-reference label to
‘\eqdef’. Then, when you refer to the equation, that text is produced.
Here's how to get the example just mentioned:

     $$...\eqdef{a-eq}$$
     ...
     $$...\eqdef[\eqrefn{a-eq}*]{a-eq-var}$$
     In \eqref{a-eq-var}, we expand on \eqref{a-eq}, ...

We use ‘\eqrefn’ in the cross-reference text, not ‘\eqref’, so that
‘\eqprint’ is called only once.

   As another example, consider the following requirement: we want to
include chapter number in all equation references, and additionally we
want to include the part number when referencing an equation from any
part other than the one where the equation appears. For example,
references to the third equation in chapter 2 of part 1 should be
typeset as '(2.3)' throughout part 1, but as '(I.2.3)' in any other
part. Let's assume we have the current chapter and part numbers in count
registers ‘\chapnum’ and ‘\partnum’, respectively.

   The idea is to have ‘\eqconstruct’ store the part number of the
equation (that is, the part number _at the time of definition_), so that
later ‘\eqprint’ can compare the stored number with the current part
number (that is, the part number _at the time of reference_). The
complicating factor is that internally, the result of ‘\eqconstruct’ is
both expanded and written out to the ‘.aux’ file, _and_ used to typeset
the equation number, so the commands that store the part number should
behave correctly in both situations. This is difficult to achieve with
expandable commands; therefore, to avoid expansion problems, we are
going to use only TeX primitives, which are non-expandable:

     \newcount\eqpartnum

     \def\eqconstruct#1{%
       \global\eqpartnum=\the\partnum\relax
       \number\chapnum.#1%
     }

     \def\eqprint#1{%
       \setbox0=\hbox{#1}%
       (\ifnum\partnum=\eqpartnum \else
          \uppercase\expandafter{\romannumeral\eqpartnum}.%
        \fi
        \box0)%
     }%

In ‘\eqconstruct’, besides constructing the base equation number (e.g.,
'1.2'), we also store the part number of the equation in the count
register ‘\eqpartnum’ (‘\the\partnum’ is expanded when the equation
number is written to the ‘.aux’ file, so the equation label definition
in the ‘.aux’ file will contain the actual part number). In ‘\eqprint’,
we need to know the equation's part number before we typeset the base
equation number, therefore we first put the argument in a box, thus
causing ‘\eqpartnum’ to be set.

4.11.2 Subequation references
-----------------------------

Eplain also provides for one level of substructure for equations. That
is, you might want to define a related group of equations with numbers
like '2.1' and '2.2', and then be able to refer to the group as a whole:
"... in the system of equations (2)...".

   The commands to do this are ‘\eqsubdef’ and ‘\eqsubdefn’. They take
one LABEL argument like their counterparts above, and generally behave
in the same way. The difference is in how they construct the equation
number: instead of using just ‘\eqnumber’, they also use another
counter, ‘\subeqnumber’. This counter is advanced by one at every
‘\eqsubdef’ or ‘\eqsubdefn’, and reset to zero at every ‘\eqdef’ or
‘\eqdefn’.

   You use ‘\eqref’ to refer to subequations as well as main equations.

   To put the two together to construct the text that the label will
produce, they use a macro ‘\eqsubreftext’. This macros takes two
arguments, the "main" equation number (which, because the equation label
can be defined as arbitrary text, as described in the previous section,
might be anything at all) and the "sub" equation number (which is always
just a number). Eplain's default definition just puts a period between
them:

     \def\eqsubreftext#1#2{#1.#2}%

You can redefine ‘\eqsubreftext’ to print however you like. For example,
this definition makes the labels print as '2a', '2b', and so on.

     \newcount\subref
     \def\eqsubreftext#1#2{%
       \subref = #2           % The space stops a <number>.
       \advance\subref by 96  % `a' is character code 97.
       #1\char\subref
     }

We must define a new count register, ‘\subref’, instead of using the
scratch count register ‘\count255’, because ‘#1’ might include other
macro calls which use ‘\count255’.

4.12 Indexing
=============

Eplain provides support for generating raw material for an index, and
for typesetting a sorted index. A separate program must do the actual
collection and sorting of terms, because TeX itself has no support for
sorting.

   Eplain can create hypertext links pointing from the index to the
index terms (see Index hyperlinks).

   Eplain's indexing commands were designed to work with the program
MakeIndex (<https://ctan.org/pkg/makeindex>); MakeIndex is also commonly
included in prepackaged TeX distributions. It is beyond the scope of
this manual to explain how to run MakeIndex, and all of its many
options.

   The basic strategy for indexing works like this:

  1. For a document ‘foo.tex’, Eplain's indexing commands (e.g., ‘\idx’;
     see the section 'Indexing terms' below) write the raw index
     material to ‘foo.idx’.

  2. MakeIndex reads ‘foo.idx’, collects and sorts the index, and writes
     the result to ‘foo.ind’.

  3. Eplain reads and typesets ‘foo.ind’ on a subsequent run of TeX. See
     the section 'Typesetting an index' below.

The ‘texi2dvi’ program can help you automate this process (see
Invoking Eplain).

   If your document needs more than one index, each must have its own
file. Therefore, Eplain provides the command ‘\defineindex’, which takes
an argument that is a single letter, which replaces ‘i’ in the filenames
and in the indexing command names described below. For example,

     \defineindex{m}

defines the command ‘\mdx’ to write to the file ‘foo.mdx’. Eplain simply
does ‘\defineindex{i}’ to define the default commands.

   Note that MakeIndex does not use the above naming scheme for multiple
indexes. Unless instructed otherwise, MakeIndex always writes its output
to a file with extension ‘.ind’. For example, if you define an
additional index with the command ‘\defineindex{j}’, you'll need to run
MakeIndex like this:

     $ makeindex book.jdx -o book.jnd

   For each index defined with ‘\defineindex{N}’, Eplain provides a
switch ‘\ifNdx’ which controls whether indexing commands write index
entries to the corresponding index file. However, even when index term
writing is disabled, indexing commands still do all other processing of
their arguments, including typesetting of proof index terms (see
Proofing index terms.

   For example, if you write ‘\idxfalse’ near the beginning of a
document ‘foo.tex’ (before the first indexing command), Eplain will not
open the default index file (‘foo.idx’) and the corresponding indexing
commands (‘\idx’, ‘\sidx’, etc.) will not write index entries there.
This may be useful for draft compilations of a manuscript, e.g., to
avoid the overhead of index file input/output.

4.12.1 Indexing terms
---------------------

Indexing commands in Eplain come in pairs: one command that only writes
the index entry to the ‘.idx’ file (see above section), and one that
also typesets the term being indexed. The former always starts with ‘s’
(for "silent"). In either case, the name always includes ‘Idx’, where I
is the index letter, also described above. Eplain defines the index ‘i’
itself, so that's what we'll use in the names below.

   The silent form of the commands take a subterm as a trailing optional
argument. For example, ‘\sidx{truth}[definition of]’ on page 75 makes an
index entry that will eventually be typeset (by default) as

     truth
       definition of, 75

   Also, the silent commands ignore trailing spaces. The non-silent ones
do not.

4.12.1.1 Indexing commands
..........................

Here are the commands.

   • ‘\sidx{TERM}[SUBTERM]’ makes an index entry for TERM, optionally
     with subterm SUBTERM. ‘\idx{TERM}’ also produces TERM as output.
     Example:

          \sidx{truth}[beauty of]
          The beauty of truth is \idx{death}.

     Subterms at the second and further levels can also be specified in
     SUBTERM, using the ‘\idxsubentryseparator’ character to separate
     them. This character is by default ‘!’.

   • ‘\sidxname{FIRST M.}{VON LAST}[SUBTERM]’ makes an index entry for
     ‘VON LAST, FIRST M.’. You can change the ‘, ’ by redefining
     ‘\idxnameseparator’. ‘\idxname{FIRST M.}{VON LAST}’ also produces
     FIRST M. VON LAST as output. (These commands are useful special
     cases of ‘\idx’ and ‘\sidx’.) Example:

          \sidxname{Richard}{Stark}
          \idxname{Donald}{Westlake} has written many kinds of novels, under
          almost as many names.

   • ‘\sidxmarked\CS{TERM}[SUBTERM]’ makes an index entry for
     ‘TERM[SUBTERM]’, but TERM will be put in the index as ‘\CS{term}’,
     but still sorted as just TERM. ‘\idxmarked\CS{TERM}’ also typesets
     ‘\CS{term}’. This provides for the usual ways of changing the
     typesetting of index entries. Example:

          \def\article#1{``#1''}
          \sidxmarked\article{Miss Elsa and Aunt Sophie}
          Peter Drucker's \idxmarked\article{The Polanyis} is a remarkable
          essay about a remarkable family.

   • ‘\sidxsubmarked{TERM}\CS{subterm}’ makes an index entry for TERM,
     SUBTERM as usual, but also puts SUBTERM in the index as
     ‘\CS{term}’. ‘\idxsubmarked{TERM}\CS{subterm}’ also typesets ‘TERM
     \CS{subterm}’, in the unlikely event that your syntax is convoluted
     enough to make this useful. Example:

          \def\title#1{{\sl #1}}
          \sidxsubmarked{Anderson, Laurie}\title{Strange Angels}
          The \idxsubmarked{Anderson}\title{Carmen} is a strange twist.

   The commands above rely on MakeIndex's feature for separating sorting
of an index entry's from its typesetting. You can use this directly by
specifying an index entry as ‘SORT@TYPESET’. For example:

     \sidx{Ap-weight@$A_\pi$-weight}

will sort as ‘Ap-weight’, but print with the proper math. The ‘@’ here
is MakeIndex's default character for this purpose. To make an index
entry with an ‘@’ in it, you have to escape it with a backslash; Eplain
provides no macros for doing this.

   After any index command, Eplain runs ‘\hookaction{afterindexterm}’.
Because the index commands always add a whatsit item to the current
list, you may wish to preserve a penalty or space past the new item. For
example, given a conditional ‘\if@aftersctnhead’ set true when you're at
a section heading, you could do:

     \hookaction{afterindexterm}{\if@aftersctnhead \nobreak \fi}

4.12.1.2 Modifying index entries
................................

All the index commands described in the previous section take an initial
optional argument before the index term, which modify the index entry's
meaning in various ways. You can specify only one of the following in
any given command, except that ‘begin’ and ‘end’ can be specified
together with ‘pagemarkup=CS’ (separate them with a comma without a
following space, like this: ‘[begin,pagemarkup=defn]’).

   These work via MakeIndex's "encapsulation" feature. See Customizing indexing,
if you're not using the default characters for the MakeIndex
operators. The other optional argument (specifying a subterm) is
independent of these.

   Here are the possibilities:

‘begin’
‘end’
     These mark an index entry as the beginning or end of a range. The
     index entries must match exactly for MakeIndex to recognize them.
     Example:

          \sidx[begin]{future}[Cohen, Leonard]
          ...
          \sidx[end]{future}[Cohen, Leonard]

     will typeset as something like

          future,
            Cohen, Leonard, 65-94

‘see’
     This marks an index entry as pointing to another; the real index
     term is an additional (non-optional) argument to the command. Thus
     you can anticipate a term readers may wish to look up, yet which
     you have decided not to index. Example:

          \sidx[see]{analysis}[archetypal]{archetypal criticism}

     becomes

          analysis,
            archetypal, see archetypal criticism

‘seealso’
     Similar to ‘see’ (the previous item), but also allows for normal
     index entries of the referencing term. The normal index entries
     have to be created separately--‘seealso’ does _not_ contribute a
     page number to the index entry. For example, if you have indexed a
     term on pages 75, 97 and 114, and then add a ‘seealso’ entry for
     the term:

          \sidx[seealso]{archetypal criticism}[elements of]{dichotomies}

     the index will contain

          archetypal criticism,
            elements of, 75, 97, 114, see also dichotomies

     (Aside for the academically curious: The archetypal critical book I
     took these dichotomous examples from is Laurence Berman's ‘The
     Musical Image’, which I happened to co-design and typeset.)

‘pagemarkup=CS’
     This puts ‘\CS’ before the page number in the typeset index, thus
     allowing you to underline definitive entries, italicize examples,
     and the like. You do _not_ precede the control sequence CS with a
     backslash. (That just leads to expansive difficulties.) Naturally
     it is up to you to define the control sequences you want to use.
     Example:

          \def\defn#1{{\sl #1}}
          \sidx[pagemarkeup=defn]{indexing}

     becomes something like

          indexing, \defn{75}

4.12.1.3 Index entries with special characters
..............................................

Indexing terms with special characters can become quite cumbersome
because you have to keep both TeX and MakeIndex happy at the same time.
For example, while ‘!’ has no special meaning for TeX, it is a subentry
separator for MakeIndex, therefore you'd have to escape occurrences of
literal ‘!’ in index terms. Things get even more interesting with
characters which are special in both TeX and MakeIndex.

   This in turn has some implications for the non-silent forms of the
indexing commands (see Indexing terms), since TeX and MakeIndex use
different conventions for escaping characters. For example, this will
not typeset the exclamation point correctly within the text, while it
will come out right inside the index, after MakeIndex strips the quoting
character (‘"’):

     \idx{"!}

This would have to be rewritten using the silent command:

     !\sidx{"!}

In general, it is a good idea to eschew the non-silent commands whenever
index term contains anything unusual.

   To understand this keep in mind that indexing commands read the terms
verbatim so that the terms can embed almost any character, and that's
what gets written into the ‘.idx’ file. The non-silent forms then
typeset the term by rescanning the verbatim copy, hence for the
non-silent commands the term, besides being a valid MakeIndex input,
must also represent a valid TeX input. The silent commands don't have
this restriction--their terms only need to become valid TeX input
_after_ MakeIndex processes the ‘.idx’ file and writes the ‘.ind’ file.
This is what makes the non-silent commands less powerful and more
troublesome when dealing with special characters.

   Here's an example showing that terms for the silent commands can
contain almost any character:

     \sidx[see]{comments}[with %@with \verbatim %"|endverbatim]
       {commenting with \verbatim %"|endverbatim}

We didn't have to escape ‘%’ in the sort string for MakeIndex, while we
had to put it inside the verbatim environment (see Verbatim listing)
in the part which MakeIndex will pass back to TeX. Also, we had to
escape the ‘|’ character because it is special for MakeIndex. If you
have trouble understanding the reasons for the different types of
escaping used, it is best to examine the ‘.idx’ and ‘.ind’ files
resulting from processing the above input.

   As was mentioned, index terms can embed "almost any character", so
now we'll describe the exceptions.

   The following characters are reset to their usual meanings because
they are not useful verbatim: multiple consecutive spaces are converted
into a single space; ASCII tab characters are treated as spaces; ASCII
return is treated as end-of-line (this means, among other things, that
long terms can be broken across several lines).

   You have to be careful with the begin- and end-group characters (‘{’
and ‘}’ by default). If they are matched, you don't have to do anything
special. For example:

     \sidx {braces {, }@braces
       \verbatim {"|endverbatim, \verbatim }"|endverbatim}

However, if they are not matched you have two problems on hand. The
first one is TeX--you have to instruct TeX to use something else as
begin- and/or end-group characters. Eplain provides an easy way to do
this: just define ‘\idxargopen’ and/or ‘\idxargclose’ to the begin- and
end-group characters you are going to use with indexing macros, and use
braces inside index terms without any restrictions. Here's an example:

     \def\idxargopen{`\<}
     \def\idxargclose{`\>}
     \sidx <left brace "{@left brace \verbatim "{"|endverbatim>

In this example we've also dealt with the second problem--braces are
MakeIndex's grouping characters as well (by default), so we have escaped
unmatched braces with ‘"’.

   And the final note: if you need a subentry containing brackets (‘[’
and ‘]’), avoid the optional argument of ‘\sidx’ and friends, and use
instead MakeIndex's subentry separator to create the subentry with the
brackets in it:

     \sidx{entry!subentry with a bracket [}

4.12.1.4 Proofing index terms
.............................

As you are reading through a manuscript, it is helpful to see what terms
have been indexed, so you can add others, catch miscellaneous errors,
etc. (Speaking from bitter experience, I can say it is extremely
error-prone to leave all indexing to the end of the writing, since it
involves adding many TeX commands to the source files.)

   So Eplain puts index terms in the margin of each page, if you set
‘\indexproofingtrue’. It is ‘false’ by default. The terms are typeset by
the macro ‘\indexproofterm’, which takes a single argument, the term to
be typeset. Eplain's definition of ‘\indexproofterm’ just puts it into
an ‘\hbox’, first doing ‘\indexprooffont’, which Eplain defines to
select the font ‘cmtt8’. With this definition long terms run off the
page, but since this is just for proofreading anyway, it seems
acceptable.

   On the other hand, we certainly don't want the index term to run into
the text of the page, so Eplain uses the right-hand side of the page
rather than the left-hand page (assuming a language read left to right
here). So ‘\ifodd\pageno’, Eplain kerns by ‘\outsidemargin’, otherwise
by ‘\insidemargin’. If those macros are undefined, ‘\indexsetmargins’
defines them to be one inch plus ‘\hoffset’.

   To get the proofing index entries on the proper page, Eplain defines
a new insertion class ‘\@indexproof’. To unbox any index proofing
material, Eplain redefines ‘\makeheadline’ to call ‘\indexproofunbox’
before the original ‘\makeheadline’. Thus, if you have your own output
routine, that redefines or doesn't use ‘\makeheadline’, it's up to you
to call ‘\indexproofunbox’ at the appropriate time.

4.12.2 Typesetting an index
---------------------------

The command ‘\readindexfile{i}’ reads and typesets the ‘.ind’ file that
MakeIndex outputs (from the ‘.idx’ file which the indexing commands in
the previous sections write). Eplain defines a number of commands that
support the default MakeIndex output.

   More precisely, ‘\readindexfile’ reads
‘\indexfilebasename.INDEX-LETTERnd’, where the INDEX-LETTER is the
argument. ‘\indexfilebasename’ is ‘\jobname’ by default, but if you have
different indexes in different parts of a book, you may wish to change
it, just as with bibliographies (see Citations).

   MakeIndex was designed to work with LaTeX; therefore, by default the
‘.ind’ file starts with ‘\begin{theindex}’ and ends with
‘\end{theindex}’. If no ‘\begin’ has been defined, Eplain defines one to
ignore its argument and set up for typesetting the index (see below),
and also defines a ‘\end’ to ignore its argument. (In a group,
naturally, since there is a primitive ‘\end’).

   Eplain calls ‘\indexfonts’, sets ‘\parindent = 0pt’, and does
‘\doublecolumns’ (see Multiple columns) at the ‘\begin{theindex}’.
‘\indexfonts’ does nothing by default; it's just there for you to
override. (Indexes are usually typeset in smaller type than the main
text.)

   It ends the setup with ‘\hookrun{beginindex}’, so you can override
anything you like in that hook (see Hooks). For example:

     \hookaction{beginindex}{\triplecolumns}

   MakeIndex turns each main index entry into an ‘\item’, subentries
into ‘\subitem’, and subsubentries into ‘\subsubitem’. By default, the
first line of main entries are not indented, and subentries are indented
1em per level. Main entries are preceded by a ‘\vskip’ of
‘\aboveitemskipamount’, ‘0pt plus2pt’ by default. Page breaks are
encouraged before main entries (‘\penalty -100’), but prohibited
afterwards--Eplain has no provision for "continued" index entries.

   All levels do the following:

     \hangindent = 1em
     \raggedright
     \hyphenpenalty = 10000

   Each entry ends with ‘\hookrun{indexitem}’, so you can change any of
this. For example, to increase the allowable rag:

     \hookaction{indexitem}{\advance\rightskip by 2em}

   Finally, MakeIndex outputs ‘\indexspace’ between each group of
entries in the ‘.ind’ file. Eplain makes this equivalent to ‘\bigbreak’.

4.12.3 Customizing indexing
---------------------------

By default, MakeIndex outputs ‘, ’ after each term in the index. To
change this, you can add the following to your MakeIndex style (‘.ist’)
file:

     delim_0 "\\afterindexterm "
     delim_1 "\\afterindexterm "
     delim_2 "\\afterindexterm "

Eplain makes ‘\afterindexterm’ equivalent to ‘\quad’.

   You can also change the keywords Eplain recognizes (see Modifying index entries):

‘\idxrangebeginword’
     'begin'

‘\idxrangeendword’
     'end'

‘\idxseecmdword’
     'see'

‘\idxseealsocmdword’
     'seealso'

   You can also change the magic characters Eplain puts into the ‘.idx’
file, in case you've changed them in the ‘.ist’ file:

‘\idxsubentryseparator’
     ‘!’

‘\idxencapoperator’
     ‘|’

‘\idxbeginrangemark’
     ‘(’

‘\idxendrangemark’
     ‘)’

   There is no macro for the ‘actual’ (‘@’ by default) character,
because it's impossible to make it expand properly.

   You can change the (imaginary) page number that "see also" entries
sort as by redefining ‘\idxmaxpagenum’. This is 99999 by default, which
is one digit too many for old versions of MakeIndex.

   The words output by Eplain for "see" and "see also" index entries are
defined by ‘\indexseeword’ and ‘\indexseealsowords’ respectively. You
can change the typeface used for these words by redefining
‘\seevariant’. And finally, the macros ‘\indexsee’ and ‘\indexseealso’
actually produce the "see ..." entries, so you can redefine them if you
want something entirely different. If you do redefine them, make them
take two parameters, the term being referenced and the ‘\idxmaxpagenum’
(the latter should normally be ignored). See the example below.

   Unfortunately, it is impossible to reliably control the commas
produced by MakeIndex in front of "see ..." entries in the ‘.ind’ file,
either at MakeIndex level or at Eplain level. However, the ‘sed’ script
contained in ‘trimsee’ distributed with Eplain in the ‘util’ directory
can be used to filter out these commas from the output of MakeIndex. For
example, suppose you want the following style for your "see ..."
entries:

     analysis,
       archetypal (see archetypal criticism)
     archetypal criticism,
       elements of, 75, 97, 114 (see also dichotomies)

You would need to redefine these macros in your TeX file:

     \def\indexsee#1#2{({\seevariant \indexseeword\/ }#1)}
     \def\indexseealso#1#2{({\seevariant \indexseealsowords\/ }#1)}

and then filter out the commas in front of the "see ..." entries by
running the following command to produce the ‘.ind’ file (assuming the
name of the ‘.idx’ file is ‘myfile.idx’ and the ‘trimsee’ script is
placed in the current directory):

     $ cat myfile.idx | makeindex | ./trimsee >myfile.ind

   By default, ‘trimsee’ uses default page list separators and default
"see ..." command names. If you set up MakeIndex to use different page
list separator or change the names of ‘\indexsee’ and ‘\indexseealso’
commands, it is possible to adjust the ‘trimsee’ script through its
command line options, which are the following:

‘-i IS’
     Use IS as a regular expression matching separator before "see ..."
     commands in the input (default: ‘, \+’).

‘-o OS’
     Use OS as a separator to replace IS before "see ..." commands
     (default: ‘ ’).

‘-s SEE’
     Use SEE as a regular expression matching "see ..." commands
     (default: ‘\\indexsee’).

‘-h’
‘--help’
     Print a usage message.

‘-v’
‘--version’
     Print version.

   ‘trimsee’ reads input from the standard input, and directs its output
to the standard output.

4.13 Justification
==================

Eplain defines three commands to conveniently justify multiple lines of
text: ‘\flushright’, ‘\flushleft’, and ‘\center’.

   They all work in the same way; let's take ‘\center’ as the example.
To start centering lines, you say ‘\center’ inside a group; to stop, you
end the group. Between the two commands, each end-of-line in the input
file also starts a new line in the output file.

   The entire block of text is broken into paragraphs at blank lines, so
all the TeX paragraph-shaping parameters apply in the usual way. This is
convenient, but it implies something else that isn't so convenient:
changes to any linespacing parameters, such as ‘\baselineskip’, will
have _no effect_ on the paragraph in which they are changed. TeX does
not handle linespacing changes within a paragraph (because it doesn't
know where the line breaks are until the end of the paragraph).

   The space between paragraphs is by default one blank line's worth.
You can adjust this space by assigning to ‘\blanklineskipamount’; this
(vertical) glue is inserted after each blank line.

   Here is an example:

     {\center First line.

        Second line, with a blank line before.
     }

This produces:

                              First line.
                Second line, with a blank line before.

   You may wish to use the justification macros inside of your own
macros. Just be sure to put them in a group. For example, here is how a
title macro might be defined:

     \def\title{\begingroup\titlefont\center}
     \def\endtitle{\endgroup}

   In addition, Eplain defines ‘\raggedleft’, analogous to plain TeX's
‘\raggedright’. This macro is also typically used inside a group, but
unlike the environments above, TeX does normal line breaking; that is,
ends-of-lines in the input file aren't treated specially. Just like
plain's ‘\raggedright’, it also resets ‘\spaceskip’ and ‘\xspaceskip’ so
that interword spacing is uniform. It also sets ‘\parfillskip’ to zero
so that last lines of paragraphs are also "ragged left". Finally,
‘\leftskip’'s new value is taken from a new glue register,
‘\raggedleft’; its default value is ‘0pt plus 2em’, the same as
‘\raggedright’'s ‘\rightskip’.

   Here's an example:

     {\raggedleft This text will be set ragged left,
     although the left margin won't be too ragged by default.
     You may well want to increase the value of
     {\tt \char`\\raggedleftskip} before calling the macro.
     It's necessary to end the paragraph before ending the group
     or the setting won't have any effect, so: {\tt \char`\\par}
     }

   Despite ‘\raggedleft’ resetting ‘\parfillskip’ to zero, TeX's line
breaking may still prefer to make the last line of a paragraph
considerably shorter than the rest, to minimize overall badness.
Increasing ‘\raggedleftskip’ may help somewhat, but using
‘\emergencystretch’, retaining interword stretchability by assigning
‘\leftskip’ directly, or even forcing line breaks may be necessary.

4.14 Tables
===========

Eplain provides a single command, ‘\makecolumns’, to make generating one
particular kind of table easier. More ambitious LaTeX styles and macro
packages tackle more difficult applications. The ‘autorows’ feature of
the Memoir package provides similar functionality to this.

   Many tables are homogenous, i.e., all the entries are semantically
the same. The arrangement into columns is to save space on the page, not
to encode different meanings. In this kind of the table, it is useful to
have the column breaks chosen automatically, so that you can add or
delete entries without worrying about the column breaks.

   ‘\makecolumns’ takes two arguments: the number of entries in the
table, and the number of columns to break them into. As you can see from
the example below, the first argument is delimited by a slash, and the
second by a colon and a space (or end-of-line). The entries for the
table then follow, one per line (not including the line with the
‘\makecolumns’ command itself).

   ‘\parindent’ defines the space to the left of the table. ‘\hsize’
defines the width of the table. So you can adjust the position of the
table on the page by assignments to these parameters, probably inside a
group.

   You can also control the penalty at a page break before the
‘\makecolumns’ by setting the parameter ‘\abovecolumnspenalty’. Usually,
the table is preceded by some explanatory text. You wouldn't want a page
break to occur after the text and before the table, so Eplain sets it to
‘10000’. But if the table produced by ‘\makecolumns’ is standing on its
own, ‘\abovecolumnspenalty’ should be decreased.

   If you happen to give ‘\makecolumns’ a smaller number of entries than
you really have, some text beyond the (intended) end of the table will
be incorporated into the table, probably producing an error message, or
at least some strange looking entries. And if you give ‘\makecolumns’ a
larger number of entries than you really have, some of the entries will
be typeset as straight text, probably also looking somewhat out of
place.

   Here is an example:

     % Arrange 6 entries into 2 columns:
     \makecolumns 6/2: % This line doesn't have an entry.
     one
     two
     three
     four
     five
     six
     Text after the table.

This produces 'one', 'two', and 'three' in the first column, and 'four',
'five', and 'six' in the second.

4.15 Margins
============

TeX's primitives describe the type area in terms of an offset from the
upper left corner, and the width and height of the type. Some people
prefer to think in terms of the “margins” at the top, bottom, left, and
right of the page, and most composition systems other than TeX conceive
of the page laid out in this way. Therefore, Eplain provides commands to
directly assign and increment the margins.

‘\topmargin = DIMEN’
‘\bottommargin = DIMEN’
‘\leftmargin = DIMEN’
‘\rightmargin = DIMEN’
     These commands set the specified margin to the DIMEN given. The ‘=’
     and the spaces around it are optional. The control sequences here
     are not TeX registers, despite appearances; therefore, commands
     like ‘\showthe\topmargin’ will not do what you expect.

‘\advancetopmargin by DIMEN’
‘\advancebottommargin by DIMEN’
‘\advanceleftmargin by DIMEN’
‘\advancerightmargin by DIMEN’
     These commands change the specified margin by the DIMEN given.

   Regardless of whether you use the assignment or the advance commands,
Eplain always changes the type area in response, not the other margins.
For example, when TeX starts, the left and right margins are both one
inch. If you then say ‘\leftmargin = 2in’, the right margin will remain
at one inch, and the width of the lines (i.e., ‘\hsize’) will decrease
by one inch.

   When you use any of these commands, Eplain computes the old value of
the particular margin, by how much you want to change it, and then
resets the values of TeX's primitive parameters to correspond.
Unfortunately, Eplain cannot compute the right or bottom margin without
help: you must tell it the full width and height of the final output
page. It defines two new parameters for this:

‘\paperheight’
     The height of the output page; default is 11truein.

‘\paperwidth’
     The width of the output page; default is 8.5truein.

   If your output page has different dimensions than this, you must
reassign to these parameters, as in

     \paperheight = 11truein
     \paperwidth = 17truein

4.16 Multiple columns
=====================

Eplain provides for double, triple, and quadruple column output: say
‘\doublecolumns’, ‘\triplecolumns’, or ‘\quadcolumns’, and from that
point on, the manuscript will be set in columns. To go back to one
column, say ‘\singlecolumn’.

   You may need to invoke ‘\singlecolumn’ to balance the columns on the
last page of output.

   To do a "column eject", i.e., move to the top of the next column, do
‘\columnfill’. This does not actually force an eject, however: it merely
inserts an unbreakable space of (essentially) size ‘\@normalvsize’ minus
‘\pagetotal’ (where ‘\@normalvsize’ is the usual height of the page; to
implement multicolumns, Eplain multiplies ‘\vsize’ itself by the number
of columns). In most circumstances, a column break will be forced after
this space (during the column splitting operation when the whole page is
output).

   The columns are separated by the value of the dimen parameter
‘\gutter’. Default value is two picas. If you want to add vertical
material between the columns, use ‘\gutterbox’. For example, to put a
vertical line between columns, define ‘\gutterbox’ as

     \def\gutterbox{\vbox to \dimen0{\vfil\hbox{\vrule height\dimen0}\vfil}}%

   There are known bugs in the multiple-column code such that ‘\topmark’
and possibly other marks can have an incorrect value on the last page of
multiple-column material when using ‘\singlecolumn’ to balance the
columns. Unfortunately this is quite difficult to fix, and at present
(volunteers welcome), it's going to remain. A suboptimal workaround is
to insert ‘\columnfill’ at the appropriate place.

The dimension counter ‘\dimen0’ contains the height of the column.

   All the ‘\...columns’ macros insert the value of the glue parameter
‘\abovecolumnskip’ before the multicolumn text, and the value of the
glue parameter ‘\belowcolumnskip’ after it. The default value for both
of these parameters is ‘\bigskipamount’, i.e., one linespace in plain
TeX.

   The macros take into account only the insertion classes defined by
plain TeX; namely, footnotes and ‘\topinsert’s. If you have additional
insertion classes, you will need to change the implementation.

   Also, Eplain makes insertions the full page width. There is no
provision for column-width insertions.

4.17 Footnotes
==============

The most common reference mark for footnotes is a raised number,
incremented on each footnote. The ‘\numberedfootnote’ macro provides
this. It takes one argument, the footnote text.

   If your document uses only numbered footnotes, you could make typing
‘\numberedfootnote’ more convenient with a command such as:

     \let\footnote = \numberedfootnote

After doing this, you can type your footnotes as ‘\footnote{FOOTNOTE
TEXT}’, instead of as ‘\numberedfootnote{FOOTNOTE TEXT}’.

   Eplain keeps the current footnote number in the count register
‘\footnotenumber’. So, to reset the footnote number to zero, as you
might want to do at, for example, the beginning of a chapter, you could
say ‘\footnotenumber=0’.

   Plain TeX separates the footnote marker from the footnote text by an
en space (it uses the ‘\textindent’ macro). In Eplain, you can change
this space by setting the dimension register ‘\footnotemarkseparation’.
The default is still an en.

   You can produce a space between footenotes by setting the glue
register ‘\interfootnoteskip’. The default is zero.

   ‘\parskip’ is also set to zero by default before the beginning of
each footnote (but not for the text of the footnote).

   You can also control footnote formatting in a more general way:
Eplain expands the token register ‘\everyfootnote’ before a footnote is
typeset, but after the default values for all the parameters have been
established. For example, if you want your footnotes to be printed in
seven-point type, indented by one inch, you could say:

     \everyfootnote = {\sevenrm \leftskip = 1in}

   By default, an ‘\hrule’ is typeset above each group of footnotes on a
page. You can control the dimensions of this rule by setting the
dimension registers ‘\footnoterulewidth’ and ‘\footnoteruleheight’. The
space between the rule and the first footnote on the page is determined
by the dimension register ‘\belowfootnoterulespace’. If you don't want
any rule at all, set ‘\footenoteruleheight=0pt’, and, most likely,
‘\belowfootnoterulespace=0pt’. The defaults for these parameters typeset
the rule in the same way as plain TeX: the rule is 0.4 points high, 2
true inches wide, with 2.6 points below it.

   The space above the rule and below the text on the page is controlled
by the glue register ‘\skip\footins’. The default is a plain TeX
‘\bigskip’.

   Eplain can create hypertext links for the footnote marks (see
Footnote hyperlinks).

4.18 Fractions
==============

Exercise 11.6 of ‘The TeXbook’ describes a macro ‘\frac’ for setting
fractions, but ‘\frac’ never made it into plain TeX. So Eplain includes
it.

   ‘\frac’ typesets the numerator and denominator in ‘\scriptfont0’,
slightly raised and lowered. The numerator and denominator are separated
by a slash. The denominator must be enclosed in braces if it's more than
one token long, but the numerator need not be. (This is a consequence of
‘\frac’ taking delimited arguments; see page 203 of ‘The TeXbook’ for an
explanation of delimited macro arguments.)

   For example, ‘\frac 23/{64}’ turns '23/64' into 23/64 (you can't see
the difference in the Info file).

4.19 Paths
==========

When you typeset long pathnames, electronic mail addresses, or other
such "computer" names, you would like TeX to break lines at punctuation
characters within the name, rather than trying to find hyphenation
points within the words. For example, it would be better to break the
email address ‘letters@alpha.gnu.ai.mit.edu’ at the ‘@’ or a ‘.’, rather
than at the hyphenation points in ‘letters’ and ‘alpha’.

   If you use the ‘\path’ macro to typeset the names, TeX will find
these good breakpoints. The argument to ‘\path’ is delimited by any
character other than ‘\’ which does not appear in the name itself. ‘|’
is often a good choice, as in:

     \path|letters@alpha.gnu.ai.mit.edu|

   You can control the exact set of characters at which breakpoints will
be allowed by calling ‘\discretionaries’. This takes the same sort of
delimited argument; any character in the argument will henceforth be a
valid breakpoint within ‘\path’. The default set is essentially all the
punctuation characters:

     \discretionaries |~!@$%^&*()_+`-=#{}[]:";'<>,.?\/|

   If for some reason you absolutely must use ‘\’ as the delimiter
character for ‘\path’, you can set ‘\specialpathdelimiterstrue’. (Other
delimiter characters can still be used.) TeX then processes the ‘\path’
argument about four times more slowly.

   The ‘\path’ macro comes from ‘path.sty’, written by Nelson Beebe and
Philip Taylor, and available at <https://ctan.org/pkg/path>.

4.20 Logos
==========

Eplain redefines the ‘\TeX’ macro of plain TeX to end with ‘\null’, so
that the proper spacing is produced when ‘\TeX’ is used at the end of a
sentence. The other ...TeX macros listed here do this, also.

   Eplain defines ‘\AMSLaTeX’, ‘\AMSTeX’, ‘\BibTeX’ ‘\eTeX’, ‘\ExTeX’,
‘\LAMSTeX’, ‘\LaTeX’, ‘\MF’, ‘\SLiTeX’, ‘\XeLaTeX’, and ‘\XeTeX’ to
produce their respective logos. (Sorry, the logos are not shown here.)
Some spelling variants of these are also supported.

   Most of these macros come from ‘texnames.sty’, compiled by Nelson
Beebe and available at
<https://mirror.ctan.org/info/biblio/texnames.sty> (part of the ‘biblio’
package, <https://ctan.org/pkg/biblio>).

4.21 Boxes
==========

The solid rectangle that Eplain uses as a marker in unordered lists
(see Lists) is available by itself: just say ‘\blackbox’.

   You can create black boxes of arbitrary size with ‘\hrule’ or
‘\vrule’.

   You can also get unfilled rectangles with ‘\makeblankbox’. This takes
two explicit arguments: the height and depth of the rules that define
the top and bottom of the rectangle. (The two arguments are added to get
the width of the left and right borders, so that the thickness of the
border is the same on all four sides.) It also uses, as implicit
arguments, the dimensions of ‘\box0’ to define the dimensions of the
rectangle it produces. (The contents of ‘\box0’ are ignored.)

   Here is an example. This small raised open box is suitable for
putting next to numbers in, e.g., a table of contents.

     \def\openbox{%
       \ht0 = 1.75pt \dp0 = 1.75pt \wd0 = 3.5pt
       \raise 2.75pt \makeblankbox{.2pt}{.2pt}
     }

   Finally, you can put a box around arbitrary text with ‘\boxit’. This
takes one argument, which must itself be a (TeX) box, and puts a printed
box around it, separated by ‘\boxitspace’ white space (3 points by
default) on all four sides. For example:

     \boxit{\hbox{This text is boxed.}}

   The reason that the argument must be a box is that when the text is
more than one line long, TeX cannot figure out the line length for
itself. Eplain does set ‘\parindent’ to zero inside ‘\boxit’, since it
is very unlikely you would want indentation there. (If you do, you can
always reset it yourself.)

   ‘\boxit’ uses ‘\ehrule’ and ‘\evrule’ so that you can easily adjust
the thicknesses of the box rules. See Rules.

4.22 Checking for PDF output
============================

You might sometimes want to test whether the target format is ‘.pdf’ or
‘.dvi’. The ‘\ifpdf’ conditional can be used for this:

     \ifpdf
        This text is produced when the engine outputs PDF.
     \else
        This text is produced when the engine outputs DVI (or similar).
     \fi

   At this writing, ‘\ifpdf’ will be true when running pdfTeX or LuaTeX
with PDF output. It will be false when running XeTeX, or (of course)
original TeX, etc.

   Eplain defines ‘\ifpdf’ by incorporating ‘iftex.sty’, a package now
maintained by the LaTeX Project Team. ‘iftex.sty’, and therefore Eplain,
defines numerous related conditionals to test for different engines; see
its package documentation for details: <https://ctan.org/pkg/ifpdf>.

4.23 Loading LaTeX packages
===========================

Eplain provides a limited support for loading LaTeX packages (‘.sty’
files--not ‘.cls’). This will mostly work for packages which were
designed with plain TeX compatibility in mind, which means that most
LaTeX packages cannot be loaded. The packages which are known to work
are listed below (see Packages known to work). If you discover a
working package which is not in the list, please report it to the Eplain
mailing list (see Introduction).

   To set up a pseudo-LaTeX environment for the packages, Eplain uses
‘miniltx.tex’ (<https://ctan.org/pkg/miniltx>) from the LaTeX graphics
collection, written by David Carlisle and Sebastian Rahtz. Eplain
extends ‘miniltx.tex’ to provide (primarily) support for package
options; in many cases, you can use ‘miniltx.tex’ directly without
loading Eplain at all.

4.23.1 The ‘\usepackage’ command
--------------------------------

‘\usepackage’ loads a LaTeX package. Its syntax is similar to that of
LaTeX's ‘\usepackage’ command:

     \usepackage[OPTIONS]{PACKAGES}[VERSION]

where OPTIONS is a comma-separated list of package options, PACKAGES is
a comma-separated list of packages to load (without the ‘.sty’ suffix),
and VERSION is a package version number given as a date in the format
‘YYYY/MM/DD’. If an older version of the package is found, a warning is
issued. If several packages are loaded within a single ‘\usepackage’
command, the OPTIONS will be applied to each of the packages. As usual,
parameters in square brackets are optional and can be omitted (together
with the square brackets).

   For example:

     \usepackage[foo,bar]{pack1,pack2}[2005/08/29]

will load packages ‘pack1’ and ‘pack2’, each with the options ‘foo’ and
‘bar’, and will check that each of the packages are dated 2005/08/29 or
newer.

4.23.2 Environment for loading packages
---------------------------------------

Some packages request that certain commands are executed after all
packages have been loaded. In LaTeX, this means that the commands are
executed at the beginning of the document, after the so-called
“preamble”. Neither plain TeX nor Eplain have a concept of preamble;
therefore, Eplain requires that all packages be loaded inside a
‘\beginpackages...\endpackages’ block. For example:

     \beginpackages
       \usepackage[foo,bar]{pack1}
       \usepackage{pack2}
     \endpackages

This requirement enables Eplain to execute the "delayed" commands at the
end of the ‘\beginpackages...\endpackages’ block.

   For the same reason, it is advisable to specify only one such block
per document, just like there is only one preamble in LaTeX.

   Both the ‘miniltx.tex’ file used by Eplain and some LaTeX packages
redefine TeX's primitive ‘\input’ to be a macro. Under plain TeX, users
probably expect the primitive ‘\input’. Therefore, at the beginning of
the ‘\beginpackages...\endpackages’ block Eplain saves the meaning of
‘\input’ as ‘\eplaininput’ and restores the original ‘\input’ at the end
of the block. This usually means that the primitive ‘\input’ is
restored, unless you (or some other macro package you've loaded
directly) have redefined it before calling ‘\beginpackages’. In case you
need to access the package-provided ‘\input’, Eplain saves it as
‘\packageinput’.

   Along the same lines, Eplain restores the catcode of ‘@’ at
‘\endpackages’ to whatever it was before (using ‘\resetatcatcode’, as
defined by ‘miniltx.tex’). This is needed because ‘miniltx.tex’, read by
‘\beginpackages’, does not restore the catcode of ‘@’, but leaves it as
11 (letter).

   Sometimes you may encounter packages which make conflicting
redefinitions of ‘\input’. Common symptoms are TeX spewing
incomprehensible error messages or hanging in a loop at a call to
‘\input’. This sometimes can be cured by restoring ‘\input’ to
‘\eplaininput’ before loading each package. For example:

     \beginpackages
       \usepackage{pack1}
       \let\input\eplaininput
       \usepackage{pack2}
     \endpackages

4.23.3 Packages known to work
-----------------------------

The following table lists packages that had been tested and are known to
work with Eplain, and locations where you can find manuals for these
packages. Some of the short descriptions of the packages were taken from
the documentation for those packages.

autopict (‘2001/06/04 v1.1j Picture mode autoload file’)
     <https://tug.org/eplain/misc/ltpictur.pdf>

     This is the LaTeX "picture mode", started by ‘\begin{picture}’ and
     ended by ‘\end{picture}’ (in LaTeX, this package is not explicitly
     loaded since it is part of the LaTeX kernel). It provides commands
     to draw simple figures inside your document without resorting to
     any external tools.

color (‘1999/02/16 v1.0i Standard LaTeX Color (DPC)’)
graphics (‘2001/07/07 v1.0n Standard LaTeX Graphics (DPC,SPQR)’)
graphicx (‘1999/02/16 v1.0f Enhanced LaTeX Graphics (DPC,SPQR)’)
     <https://ctan.org/pkg/graphics>

     These packages are from the LaTeX graphics collection. (The
     independent ‘xcolor’ package does not work with Eplain.) They
     provide commands for changing text/page colors, text rotation and
     scaling, and much more.

     *Warning 1:* If you encounter problems loading one of these
     packages under pdfTeX (when pdfTeX reads ‘supp-mis.tex’), the cause
     may be an outdated ‘supp-mis.tex’ (part of ConTeXt, a typesetting
     system for TeX) installed on your system. The problem was fixed in
     ‘supp-mis.tex’ version 2004.10.26. You can obtain up-to-date
     versions of ‘supp-mis.tex’ and the accompanying ‘supp-pdf.tex’ from
     <https://mirror.ctan.org/macros/pdftex/graphics>. To convince TeX
     to use the new files, you have the following options:

       1. put the new files in the same directory with your document;

       2. overwrite the outdated files installed by your TeX
          distribution;

       3. install the new ‘supp-mis.tex’ and ‘supp-pdf.tex’ files in the
          relevant subdirectory of your local ‘texmf’ tree (for info on
          TeX directory structure, see <https://tug.org/tds/>);

       4. upgrade your ConTeXt installation.

     Note that option 1 is the safest but provides the fix only for your
     current document. Options 2 and 3 will usually suffice for Eplain
     but may break ConTeXt. Option 4 is the most general but is more
     complicated than the first three. Be sure to backup any files you
     overwrite. Also keep in mind that upgrading your TeX distribution
     may overwrite files you install in the system ‘texmf’ tree.
     *End of warning 1.*

     *Warning 2:* If you encounter problems using the ‘\pagecolor’
     command from the ‘color.sty’ package under pdfTeX, the cause may be
     an outdated pdfTeX color and graphics driver ‘pdftex.def’. The
     problem was fixed in ‘pdftex.def’ version 0.03p. You can obtain an
     up-to-date version from <https://ctan.org/pkg/graphics-def>.
     *End of warning 2.*

     The ‘\fcolorbox’ macro provided by the ‘color’ package requires the
     macro ‘\fbox’ to work, but ‘miniltx’ does not provide that. Here is
     a definition for it that uses Eplain's ‘\boxit’ (see Boxes),
     thanks to Dan Luecking and Helmut Jarausch:

          \makeatletter
          \def\fbox#1{{%
            \hruledefaultheight=\fboxrule
            \hruledefaultdepth=0pt
            \vruledefaultwidth=\fboxrule
            \let\boxitspace\fboxsep % use miniltx register
            \boxit{\color@begingroup\hbox{#1}\color@endgroup}%
          }}
          \makeatother

     The ‘graphics’/‘graphicx’ packages have the option ‘draft’ which
     instructs ‘\includegraphics’ not to include the graphics but
     instead typeset a box with the dimensions of the graphics and the
     name of the graphics file in typewriter type at the center of the
     box. These packages expect the LaTeX-provided command ‘\ttfamily’
     to switch to typewriter type. This command is not defined by
     ‘miniltx.tex’, therefore Eplain defines it and makes it equivalent
     to plain TeX's ‘\tt’.

     See Hyperlinks (xhyper.tex), for the demonstration of text
     rotation and graphics inclusion using the ‘graphicx’ package, and
     using the ‘color’ package to colorize hypertext links.

     Klaus Höppner has written a nice introduction to the LaTeX graphics
     packages and different graphics formats. You can download it from
          <https://tug.org/pracjourn/2005-3/hoeppner>

epstopdf (‘2009/07/16 v2.2 Conversion with epstopdf on the fly (HO)’)
     <https://ctan.org/pkg/epstopdf-pkg>

     This package does on-the-fly conversion of Encapsulated PostScript
     (EPS) graphics into Portable Document Format (PDF) graphics for
     inclusion with the ‘\includegraphics’ command from the
     ‘graphics’/‘graphicx’ packages, so that you do not have to
     explicitly call the ‘epstopdf’ script.

psfrag (‘1998/04/11 v3.04 PSfrag (MCG)’)
     <https://ctan.org/pkg/psfrag>

     PSfrag allows the user to precisely overlay Encapsulated PostScript
     (EPS) files with arbitrary (La)TeX constructions. In order to
     accomplish this, the user places a simple text "tag" in the
     graphics file, as a "position marker" of sorts. Then, using simple
     (La)TeX commands, the user instructs PSfrag to remove that tag from
     the figure, and replace it with a properly sized, aligned, and
     rotated (La)TeX equation.

soul (‘2003/11/17 v2.4 letterspacing/underlining (mf)’)
     <https://ctan.org/pkg/soul>

     This package provides hyphenatable letterspacing (spacing out),
     underlining, and some derivatives. The package is optimized for
     LaTeX, but works with plain TeX--you don't actually need to load it
     with the ‘\usepackage’ command, just say ‘\input soul.sty’. If you
     intend to use the highlighting macros of ‘soul’, don't forget to
     load the ‘color’ package.

url (‘2005/06/27 ver 3.2 Verb mode for urls, etc.’)
     <https://ctan.org/pkg/url>

     This package provides a form of ‘\verbatim’ that allows line breaks
     at certain characters or combinations of characters, accepts
     reconfiguration, and can usually be used in the argument to another
     command. It is intended for email addresses, hypertext links,
     directories/paths, etc., which normally have no spaces.

     Eplain can create hypertext links with the ‘\url’ command (see
     URL hyperlinks).

     Be sure to get a version dated at least 2005/06/27, as older
     versions have problems in plain TeX.

4.23.4 Packages known not to work
---------------------------------

The following packages are known not to work with Eplain:

hyperref
     <https://ctan.org/pkg/hyperref>

     This package depends heavily on LaTeX, so that it is essentially
     unusable outside of LaTeX. Eplain provides its own macros for
     creating hyperlinks; see Hyperlinks.

microtype (‘2013/05/23 v2.51 Micro-typographical refinements (RS)’)
     <https://ctan.org/pkg/microtype>

pict2e (‘2005/07/15 v0.2r Improved picture commands (HjG,RN)’)
     <https://ctan.org/pkg/pict2e>

xcolor (‘2005/06/06 v2.03 LaTeX color extensions (UK)’)
     <https://ctan.org/pkg/xcolor>

5 Hyperlinks
************

This chapter describes the support which Eplain provides for hypertext
links (“hyperlinks” for short). Hyperlinks can be created implicitly by
the cross-reference, indexing and other macros in Eplain. Macros for
constructing explicit hyperlinks are also provided.

5.1 Introduction to hyperlinks
==============================

The original TeX engine has no built-in support for hyperlinks (a.k.a.
hypertext links). Many of the present-day file formats with hyperlinking
capabilities did not even exist at the time TeX was written. However,
TeX's ‘\special’ primitive can be used to instruct TeX to write special
directives into its ‘.dvi’ output file. These directives are not
interpreted by TeX in any way; they are intended for programs which
process the ‘.dvi’ files produced by TeX, be it printing or converting
to other formats, such as ‘.ps’ or ‘.pdf’.

   Another approach is to extend the original TeX engine with the
ability to generate one of the hyperlinking formats; TeX's set of
primitives can be extended to include hyperlink commands. This is the
approach used by the pdfTeX engine, which is capable of producing ‘.pdf’
files directly from the TeX source, skipping the ‘.dvi’ generation and
processing step.

   It turns out that the sets of commands for different formats are
mostly not interchangeable, as each of the file formats has its own
quirks and capabilities. And this is where Eplain “hyperlink drivers”
come into play.

   In order for Eplain to generate proper commands, Eplain has to know
two things: which engine or ‘.dvi’ processor you are using, and the set
of commands it understands.

   The knowledge about the commands that the various processors
understand is programmed into Eplain's hyperlink drivers. Eplain
provides three drivers: ‘hypertex’ (implementation of the HyperTeX
standard, see <https://arxiv.org/hypertex>), and ‘pdftex’ and ‘dvipdfm’
(named after the programs which process the hyperlink commands, pdfTeX
and dvipdfm). Therefore, Eplain can only produce HyperTeX commands and
hyperlink commands for one of these two programs--except that the
extended ‘dvipdfmx’ program can be used as well as the original
‘dvipdfm’, since they are compatible.

   To tell Eplain which ‘.dvi’ processor or extended TeX engine you are
using, use the command ‘\enablehyperlinks’.

   For example:

     \enablehyperlinks

instructs Eplain to attempt to automatically detect which driver to use,
as follows: if it detects pdfTeX in PDF mode, it loads the ‘pdftex’
driver. If it does not detect pdfTeX in PDF mode, the ‘hypertex’ driver
is loaded. The detection is based on the ‘\ifpdf’ switch (see Checking for PDF output).

   If necessary, you can explicitly specify the driver name:

     \enablehyperlinks[dvipdfm]

will start producing hyperlinks under the assumption that you are using
pdfTeX.

   Eplain does not produce any hyperlinks until you explicitly enable
them with ‘\enablehyperlinks’. For one thing, this keeps Eplain
backward-compatible with previous releases without hyperlink support.
For another, you may be using a program other than pdfTeX or ‘dvipdfm’,
which does not understand their hyperlink commands or the HyperTeX
commands.

Concepts and Terminology
------------------------

In general, hyperlinks work as follows. You mark some place in your
document as a hyperlink destination, associating a “hyperlink label”
with that destination. Next, somewhere within your document, you create
a hyperlink, using a label to identify the destination you want this
link to point to. A hyperlink is a region in the document (which can
take many forms, for example, text or a picture); when a user clicks on
that region, they will be taken to a place in the document marked by the
corresponding destination. The following two sections (see Explicit hyperlinks,
and Implicit hyperlinks) describe the macros you
can use to define destinations and create links pointing to those
destinations.

   In the rest of this chapter, we will often need to refer to links and
destinations jointly, in which case we will use the term “hyperlinks”.
We will use the terms “links” and “destinations” in cases when we need
to refer specifically to links or destinations.

   Hyperlink drivers provide several kinds of links and destinations. We
will refer to them as “link types” and “destination types”.

   For example, one of the destination types that the ‘pdftex’ driver
provides is the ‘xyz’ type; when the user follows a link pointing to an
‘xyz’ destination, the exact location marked by that destination is
displayed. Another destination type provided by the ‘pdftex’ driver is
the ‘fit’ type; when the user follows a link pointing to a ‘fit’
destination, the page containing that destination is zoomed to fit into
the window in which the document is displayed.

   Similarly, drivers support various link types. For example, with the
‘pdftex’ driver, the usual link type used to refer to destinations in
the current document is called ‘name’. You can also create a link
pointing to another local document (by using the ‘filename’ link type)
or to a url (by using the ‘url’ link type).

   In addition, each hyperlink driver supports a number of destination
and link “options”. By setting these options you can customize hyperlink
parameters (e.g., the thickness of the border drawn around a hyperlink)
or pass information to hyperlinks (for example, file name of a document,
for a link pointing to a destination in another document).

   See Hyperlink drivers, for the description of hyperlink types and
options supported by the drivers. See Setting hyperlink types and options,
for the information on how to set hyperlink options.

5.2 Explicit hyperlinks
=======================

Explicit hyperlinks are created by you, in the source of your document.
The simplest command is ‘\hldest’, which marks the current position in
your document as a destination:

     \hldest{TYPE}{OPTIONS}{LABEL}

Here TYPE is one of the destination types supported by the hyperlink
driver (see Hyperlink drivers), OPTIONS is a comma-separated list of
option assignments, and LABEL is the hyperlink label to associate with
this destination. This label will identify the destination when creating
links pointing to this destination. For example, with the ‘pdftex’
driver, the command

     \hldest{xyz}{zoom=2000}{index}

creates a destination of type ‘xyz’ ("the current position"), sets the
magnification ratio for this destination to be 200%, and associates the
label ‘index’ with the destination.

   Another command, ‘\hlstart’, paired with ‘\hlend’, turns all
intervening material into a link:

     \hlstart{TYPE}{OPTIONS}{LABEL} ... \hlend

Here TYPE, OPTIONS and LABEL have the same meaning as for ‘\hldest’.
Continuing the previous example,

     \hlstart{name}{bstyle=U,bwidth=2}{index} Index\hlend

typesets the word 'Index' as a link with underline border of
width 2 PostScript points, pointing to the named destination ‘index’
defined in the previous example. (The other options, like highlight mode
and border color, are determined by the defaults, see Setting default types and options).

   The LABEL argument of both ‘\hldest’ and ‘\hlstart’ can contain
special characters (such as ‘#’, ‘%’, ‘&’, ‘~’, etc.) without any
escaping. This is especially important for url links supported by some
drivers (see Hyperlink drivers).

   Both ‘\hldest’ and ‘\hlstart’ ignore following spaces.

   Both ‘\hldest’ and ‘\hlstart’ expand the first token of OPTIONS once,
so you can save a list of options in a macro and pass it for the
OPTIONS. For example:

     \def\linkopts{bstyle=U,bwidth=2}
     \hlstart{name}{\linkopts}{index}Index\hlend

is functionally equivalent to the previous example.

   See Hyperlinks (xhyper.tex), for a demonstration of the usage of
explicit hyperlinks.

5.3 Implicit hyperlinks
=======================

“Implicit hyperlinks” are hyperlinks created implicitly by various
Eplain macros, such as the macros for citations, cross-references,
indexing, etc.

   All such macros are divided into “link groups” and “destination
groups” (or “linkgroups” and “destgroups” for short) so that parameters
can be set individually for each group. For example, all equation macros
which define a destination are assigned to the 'eq' destgroup; equation
macros which create a link are assigned to the 'eq' linkgroup. By
setting parameters for the 'eq' linkgroup (destgroup), you can uniformly
customize all links (destinations) related to equation references,
without interfering with settings for the other groups.

   See Setting hyperlink types and options, for information on how
to set parameters for a group.

   Here is the list of the linkgroups:

     hrefint, hrefext, url, cite, ref, xref, eq, idx, foot, footback.

   And here are the destgroups:

     bib, li, definexref, xrdef, eq, idx, foot, footback.

   See Hyperlinks (xhyper.tex), for a demonstration of the usage of
implicit hyperlinks.

   The following subsections describe each of the linkgroups and
destgroups and the hyperlink support provided.

5.3.1 General hyperlinks: hrefint, hrefext
------------------------------------------

‘\href{URL}{TEXT}’ typesets TEXT as a link to URL. It basically does
what the explicit hyperlink macros do (see Explicit hyperlinks), but
is more convenient (at the expense of flexibility).

   If URL starts with ‘#’, the rest of URL is assumed to be a local
hyperlink destination name (destination within the same document).
Parameters for these links can be set by customizing the 'hrefint'
linkgroup. For example:

     See \href{#intro}{Introduction}

will make 'Introduction' into an internal link, which might have been
created, e.g., with ‘\xrdef{intro}’.

   If URL does not start with ‘#’, it is assumed to be a url link.
Parameters for these links can be set by customizing the 'hrefext'
linkgroup. The special characters (such as ‘#’ and ‘~’) in the url don't
need to be escaped. For example:

     \href{https://tug.org/eplain/doc/eplain.html#Hyperlinks}{Hyperlinks
       in Eplain}
     \href{mailto:tex-eplain@tug.org}{Eplain mailing list}

See URL hyperlinks, for another way to create url hyperlinks.

   The TEXT argument of ‘\href’ can contain verbatim text (see
Verbatim listing) or other macros which manipulate character catcodes.
For example:

     \href{#WeirdChars}{The weird chars \verbatim #&%$~|endverbatim}

   ‘\href’ does not currently handle other link types, such as ‘file:’
and ‘run:’ links.

5.3.2 URL hyperlinks: url
-------------------------

The 'url' linkgroup covers the ‘\url’ command from the LaTeX package
‘url’ (see Packages known to work), as well as any new ‘\url’-like
commands you define. The type for this linkgroup is set to ‘url’ by the
drivers which support this link type. ‘url’ links use the parameter to
the ‘\url’ command as the url to point to.

   You may be using the ‘\url’ command to typeset something other than a
url, e.g., a path, for which you do not want a link to be created; in
that case, you can disable the 'url' linkgroup with the command
‘\hloff[url]’ (see Turning hyperlinks on/off for a group).

   By default, url (and other types of) links are boxed, so that they
are visually marked even if you do not load the LaTeX 'color' package
(see Loading LaTeX packages) and therefore link text is not colored.
You can see the effect by compiling the following code snippet (be sure
to get a modern ‘url.sty’, older versions do not work in plain TeX;
see Packages known to work):

     \input eplain
     \beginpackages
       \usepackage{url}
     \endpackages

     \enablehyperlinks

     \url{https://foo/bar}

     \url{mailto:foobar@example.org}

     \bye

   If the hyperlink driver you use supports the link option ‘bwidth’
(see Hyperlink drivers), you can produce colored links with no
border around them. Try this:

     \input eplain
     \beginpackages
       \usepackage{url}
       \usepackage{color}
     \endpackages

     \enablehyperlinks
     \hlopts{bwidth=0}

     \url{https://foo/bar}

     \url{mailto:foobar@example.org}

     \bye

The command ‘\hlopts{bwidth=0}’ sets border width to zero as the default
for all links, and loading the ‘color’ package automatically colors
links using the default color (see Options supported by all drivers).
If you want the border width setting to apply to url links
only, say ‘\hlopts[url]{bwidth=0}’ (see Setting hyperlink types and options).

5.3.3 Citation hyperlinks: cite, bib
------------------------------------

The 'cite' linkgroup includes only the ‘\cite’ command (see
Citations). ‘\cite’ turns each of the references in the list into a
link pointing to the respective bibliography entry produced by the
‘\bibliography’ command.

   The 'bib' destgroup includes the macros related to the
‘\bibliography’ command (see Citations). ‘\bibliography’ inputs a
‘.bbl’ file, which contains a list of bibliography entries. For each of
the entries, a destination is defined.

   Both commands use the citation label as the hyperlink label.

5.3.4 List hyperlinks: li
-------------------------

The 'li' destgroup consists of the ‘\li’ command (see Lists), which
defines a destination if you supply the optional argument
(cross-reference label). This label is also used as the hyperlink label.

5.3.5 Cross-reference hyperlinks: definexref, ref
-------------------------------------------------

The 'definexref' destgroup is for the ‘\definexref’ command (see
Defining generic references). ‘\definexref’ defines a destination
using the cross-reference label (the first argument) as the hyperlink
label.

   The 'ref' linkgroup includes ‘\refn’ and ‘\xrefn’ (they are
synonyms), ‘\ref’, and ‘\refs’ (see Using generic references).

   ‘\refn’ turns the cross-reference it produces into a link, using the
cross-reference label as the hyperlink label. If an optional argument is
present, it is tied by ‘\reftie’ to the reference and become part of the
link.

   ‘\ref’ works similarly to ‘\refn’. It takes an optional argument,
which is treated the same way as the optional argument to ‘\refn’. In
addition, ‘\ref’ can produce a "class word". Both the optional argument
and the class word become part of the link, when present. The
cross-reference is tied by ‘\reftie’ to the preceding word. The optional
argument is separated from the class word by ‘\refspace’.

   Unlike ‘\ref’, ‘\refs’ does not take an optional argument and does
not make the class word part of the link, which is appropriate for its
intended use.

5.3.6 Page reference hyperlinks: xrdef, xref
--------------------------------------------

The 'xrdef' destgroup is for ‘\xrdef’ (see Page references).
‘\xrdef’ defines a destination using cross-reference label as the
hyperlink label.

   The 'xref' linkgroup includes the ‘\xref’ command (see Page references).
‘\xref’ turns its optional argument (followed by
‘\refspace’), ‘\xrefpageword’ and the cross-reference (page number) into
a link, using the cross-reference label as the hyperlink label.

5.3.7 Equation reference hyperlinks: eq
---------------------------------------

All commands that define equation labels are part of the 'eq' destgroup.
These are ‘\eqdef’, ‘\eqdefn’, ‘\eqsubdef’ and ‘\eqsubdefn’ (see
Equation references). All these commands use the equation label as the
hyperlink label. However, if the equation label is empty, they make up a
(hopefully) unique hyperlink label for the destination. This label will
be used for the link when you refer to this empty-labeled equation with
one of the equation reference macros.

   The command ‘\phantomeqlabel’ is called to generate hyperlink labels
for the empty-labeled equations. By default, it produces the labels in
the format ‘PHEQNUMBER’, where NUMBER comes from the count register
‘\phantomeqnumber’; this count register is incremented at every
empty-labeled equation definition.

   The commands ‘\eqref’ and ‘\eqrefn’ (see Equation references)
form the 'eq' linkgroup. These commands take an optional argument,
which, when present, is tied with ‘\reftie’ to the equation reference
and becomes part of the link. The equation label is used for the
hyperlink label; if the label is empty, the link is for the label
generated for the last empty-labeled equation.

5.3.8 Index hyperlinks: idx
---------------------------

All indexing commands (‘\idx’, ‘\idxname’, ‘\idxmarked’, ‘\idxsubmarked’
and their silent equivalents, see Indexing commands) form the 'idx'
destgroup. The 'idx' linkgroup consists of the macros which are used to
typeset the index when you say ‘\readindexfile{INDEX-LETTER}’ (see
Typesetting an index).

   To create the links in index entries, Eplain uses MakeIndex's
"encapsulation" feature. When you use an indexing macro to mark an index
term, Eplain writes out a line to the ‘.idx’ file of the following
general form:

     \indexentry{ENTRY|PAGEMARKUP}{PAGENO}

where ENTRY is the index entry (converted into the internal format that
MakeIndex understands), CS is the markup command you specified with the
‘pagemarkup=CS’ optional argument to the indexing commands (see
Modifying index entries), and PAGENO is the page number on which the
term appeared. When processing the ‘.idx’ file, MakeIndex makes the page
number an argument to the page markup command ("encapsulates" the page
number), so the page number in the ‘.ind’ file appears as ‘\CS{PAGENO}’.
Eplain internally replaces the CS command name with its own command,
which, in addition to calling the original ‘\CS’ encapsulator, turns the
page number into a link.

   Eplain provides two approaches to linking page numbers in the index
to locations of index terms in the text.

5.3.8.1 Exact destinations for index terms
..........................................

In this approach, each command that marks an index term defines a unique
destination and passes its label on to the ‘.idx’ file as part of the
‘\indexentry’ command. The ‘\indexentry’ line that Eplain writes to the
‘.idx’ file becomes

     \indexentry{ENTRY|hlidx{LABEL}{CS}}{PAGENO}

where ‘\hlidx’ is the command that is defined by Eplain to take three
arguments: a hyperlink label (LABEL), a name of page number encapsulator
(CS) and a page number (PAGENO). In the ‘.ind’ file that MakeIndex will
generate, the page number will now appear as

     \hlidx{LABEL}{CS}{PAGENO}

The result of this command is ‘\CS{PAGENO}’, wrapped up into a link
pointing to LABEL destination.

   The hyperlink labels for the index terms are generated by the
‘\hlidxlabel’ command, by default in the format ‘IDXNUMBER’, where
NUMBER is the value of the count register ‘\hlidxlabelnumber’. This
count register is incremented at each index term.

   The advantage of this approach, as compared to the second approach
described below, is that links in the index point to exact locations of
the indexed terms on the page. The disadvantage of this approach is that
MakeIndex will regard _all_ index entries as distinct, because each one
contains a (unique) hyperlink label. This disadvantage can be partially
overcome by the script ‘idxuniq’ distributed with Eplain in the ‘util’
directory. This script filters out ‘\indexentry’ lines differing only in
the hyperlink label but identical otherwise. You should process the
‘.idx’ with this script before passing it on to MakeIndex. For example:

     $ ./idxuniq file.idx | makeindex >file.ind

Still, this solution is not ideal, as the page-range formation ability
of MakeIndex will not work, and there will be problems of apparently
identical index entries clashing (e.g., when a range-end entry appears
on the same page as another entry with the same definition; ‘idxuniq’
will not filter out the second entry).

5.3.8.2 Page destinations for index terms
.........................................

In the second approach, Eplain does not write out any destination labels
for the index terms. Instead, Eplain writes out a wrapper for page
number encapsulator which can parse the page number and generate a link
pointing to the _page_ on which the term appeared. On top of each page
containing an index term, Eplain defines a destination with label
produced by ‘\hlidxpagelabel’. The ‘\hlidxpagelabel’ command takes a
single argument (page number NUMBER) and by default produces the label
in the format ‘IDXPGNUMBER’.

   With this approach, the ‘\indexentry’ line which Eplain writes to the
‘.idx’ file looks like this:

     \indexentry{ENTRY|hlidxpage{CS}}{PAGENO}

where ‘\hlidxpage’ is the command that is defined by Eplain to take two
arguments: a name of page number encapsulator (CS) and a page number
(PAGENO). In the ‘.ind’ file that MakeIndex will generate, the page
number will appear as

     \hlidxpage{CS}{PAGENO}

   The advantage of this approach is that all features of MakeIndex are
intact. The drawback is that links in the index do not point to exact
locations of indexed terms on a page, but to the top of a page on which
the term appears.

   Another disadvantage is that this approach depends on the page range
and page list separators which MakeIndex was configured to output.
‘\hlidxpage’ must be able to parse the first page number in a page range
like ‘1--4’. In addition, page list parsing is needed because MakeIndex
combines two consecutive page numbers in one call to the page number
encapsulator, so ‘\hlidxpage’ can be passed, e.g., ‘1, 2’ for the
PAGENO. In this last case, ‘\hlidxpage’ splits the two page numbers,
applies ‘\CS’ to each of them, and makes each of the page numbers a link
to the appropriate page. Note that this will alter typesetting slightly,
because now the page list separator (a comma followed by a space, by
default) is not typeset using the page number encapsulator (‘\CS’).

   Eplain's defaults for the page list and page number delimiters are
the same as those in MakeIndex, a comma followed by a space (‘, ’) and
two dashes (‘--’), respectively. If you customize MakeIndex to use
different delimiters, you must not forget to let Eplain know about them
with the commands

     \setidxpagelistdelimiter{LIST-DELIM}
     \setidxpagerangedelimiter{PAGE-DELIM}

(see Page list and page range parsers).

5.3.8.3 Choosing destination placement
......................................

The approach that Eplain should use for the index terms can be selected
in the ‘\enablehyperlinks’ command. The optional argument it accepts is
a comma-separated list of options. The ‘idxexact’ option selects the
first approach, ‘idxpage’ the second, and ‘idxnone’ disables hyperlink
support for the index terms altogether, in case you want to stop Eplain
from writing its link wrappers into the ‘.idx’ file. The default is
‘idxpage’.

   For example:

     \enablehyperlinks[idxexact]

selects the first approach ("exact index links").

5.3.8.4 Index page list and page range parsers
..............................................

The macros that Eplain uses to parse page lists and page ranges,
‘\idxparselist’ and ‘\idxparserange’, can sometimes be useful when
defining page number encapsulators. See Page list and page range parsers,
for the description of these commands and an example of their
usage.

5.3.8.5 Hyperlinks in see and see also entries
..............................................

There is no automatic support for hyperlinks with "see" and "see also"
index entries, as there is not enough information to trace the
parameters of ‘\indexsee’ and ‘\indexseealso’ to corresponding index
entries. But if desired, this can be implemented with ‘\hldest’ and
‘\hlstart’ (see Explicit hyperlinks); for example:

     \sidx{semantic theory of truth@%
           \leavevmode\hldest{}{}{idx:theo truth}semantic theory of truth}
     ...
     \sidx[seealso]{truth}[definition of]%
           {\hlstart{}{}{idx:theo truth}semantic theory of truth\hlend}

5.3.9 Footnote hyperlinks: foot, footback
-----------------------------------------

The 'foot' link and destination groups include the ‘\numberedfootnote’
and ‘\footnote’ macros (see Footnotes). The 'footback' groups
include the same macros, but control parameters for links and
destinations created inside the footnote to point back to the footnote
mark within the text body.

   The macros use hyperlink labels generated by ‘\hlfootlabel’ and
‘\hlfootbacklabel’. The default formats for the labels are ‘FOOTNUMBER’
and ‘FOOTBNUMBER’, respectively, where NUMBER is the value of the count
register ‘\hlfootlabelnumber’. This register is incremented at every
footnote.

   Generally, footnote hyperlinks are not of much use, because the
footnotes are usually placed on the same page as the footnote mark.
Therefore, footnote hyperlinks are disabled by default. Here is how you
can turn them on selectively, without affecting the other kinds of
hyperlinks (see Turning hyperlinks on/off for a group):

     \hldeston[foot,footback]
     \hlon[foot,footback]

5.3.10 Contents hyperlinks
--------------------------

There is currently no special support for hyperlinks in the table of
contents (see Contents), but implementing them with the ‘\hldest’
and ‘\hlstart ... \hlend’ commands (see Explicit hyperlinks) should
be possible.

5.4 Hyperlink drivers
=====================

This section describes the hyperlink drivers: the types of hyperlinks
they support, and the options they accept. During the first reading, you
may only want to skim through this section.

   Some of the descriptions below come from ‘Portable Document Format
Reference Manual Version 1.3’, March 11, 1999.

5.4.1 Options supported by all drivers
--------------------------------------

This subsection describes the destination and link options which are
supported by all hyperlink drivers.

Destination options supported by all drivers
............................................

‘raise’
     Specifies how much to raise destinations above the baseline. When
     set to zero or empty, destinations are placed at the baseline.

     It is usually convenient to set this option to some variable
     parameter, so that the height to which destinations are raised is
     automatically adjusted according to the current context. For
     example, setting it to ‘\normalbaselineskip’ (or some fraction of
     it, like ‘1.7\normalbaselineskip’) makes the setting appropriate
     for different point sizes, in case your document uses more than
     one.

     The default setting is ‘\normalbaselineskip’. Initially, the
     destgroups do not define this option, so they fall back on the
     default, except for the 'eq' destgroup, for which this option is
     set to ‘1.7\normalbaselineskip’, to accommodate the usual cases of
     large operators in displayed math.

     Example: ‘\hldestopts[eq]{raise=2.5\normalbaselineskip}’

Link options supported by all drivers
.....................................

‘colormodel’
‘color’
     These two options define the color to be used for rendering the
     link text. The colors are used only when a ‘\color’ command is
     defined, e.g., by loading the LaTeX 'color' package (see Packages known to work).
     The ‘\color’ command is called as
     ‘\color[COLORMODEL]{COLOR}’, where COLORMODEL and COLOR are the
     definitions of the ‘colormodel’ and ‘color’ options, respectively.
     However, if COLORMODEL is empty, the optional argument to ‘\color’
     is omitted; and if COLOR is empty, the ‘\color’ command is omitted
     altogether. The default setting is ‘COLORMODEL=cmyk’ and
     ‘COLOR=0.28,1,1,0.35’.

     When specifying colors with several components delimited by commas
     (e.g., RGB and CMYK colors in the LaTeX 'color' package), it is not
     possible to specify the components directly in the option list of
     ‘\hlopts’, because comma is the option list delimiter. With the
     'color' package, it is possible to specify such colors by defining
     a custom color with ‘\definecolor’ and using the new color name
     with an empty COLORMODEL (see examples below).

     Examples:

          \hlopts{colormodel=,color=blue}% predefined color
          \definecolor{mycolor}{rgb}{.3,.8,.95}
          \hlopts{colormodel=,color=mycolor}% custom color
          \hlopts{colormodel=gray,color=.4}

5.4.2 Hyperlink driver ‘hypertex’
---------------------------------

HyperTeX is a standard for inclusion of hyperlink information in TeX
(and LaTeX) documents (see <https://arxiv.org/hypertex>). This standard
defines a set of hyperlink tags implemented as ‘\special’ commands
written into the DVI file. The major advantage of such standard is that
a single ‘.dvi’ file containing HyperTeX commands can be viewed by any
HyperTeX-enabled viewer (e.g., any more or less modern version of
‘xdvi’) or converted to other file formats (e.g., PDF) by any
HyperTeX-enabled DVI converter (e.g., ‘dvipdfmx’ or ‘dvips’ with
Ghostscript's ‘ps2pdf’ script).

   The downside to the standard is that it is by design "the common
factor" of other formats supporting hyperlinks, so many features of a
particular file format cannot be supported by HyperTeX. Therefore, if
you need to use special features of a particular format, HyperTeX is not
a good choice. For the PDF file format, Eplain provides the ‘pdftex’ and
‘dvipdfm’ drivers which provide fine control over the PDF options (see
Hyperlink drivers pdftex and dvipdfm).

   For more information on programs which support the HyperTeX standard,
please see

     <https://arxiv.org/hypertex>
     <https://mirror.ctan.org/support/hypertex/hypertex>

For convenience, we list a few HyperTeX-enabled converters:

‘dvips’
     Note that you need to pass the ‘-z’ option to ‘dvips’ to tell it to
     preserve the information about the hyperlinks. To generate a ‘.pdf’
     file with hyperlinks, you can use the ‘ps2pdf’ script of
     Ghostscript. For example, if ‘foo.tex’ is a TeX file using HyperTeX
     commands, then

          $ tex foo.tex
          $ dvips -z foo.dvi -o
          $ ps2pdf foo.ps

     will produce ‘foo.pdf’ with hyperlinks.

‘dvipdfm’
‘dvipdfmx’
     No special command line arguments are required, these programs
     automatically detect the HyperTeX commands.

   One more note is in place: support for the HyperTeX commands varies
from one previewer/converter to another, so you might need to experiment
to see if what you need is supported by the program(s) you intend to
use. For example, dvips(k) as of version 5.92b converts all internal
hyperlinks into page links pointing to a page containing the
destination, so that "exact" linking to destinations is not possible
(this was confirmed to be a bug and most probably has already been fixed
in later versions of dvips(k)); dvipdfm as of version 0.13.2c and
dvipdfmx as of version 20040411 do not correctly parse links to external
local files, and produce a url link instead of a file link.

5.4.2.1 Destination types for ‘hypertex’
........................................

‘xyz’
     "Current position". This is the default type.

     Example: ‘\hldest{xyz}{}{dest123}’

‘raw’
     The destination specification (in the form of a HyperTeX
     ‘\special’) is taken from the command sequence ‘\CS’, where CS is
     the value of the ‘cmd’ option. In the definition of ‘\CS’, use
     ‘\@hllabel’ to refer to the hyperlink label. This option is
     intended to be used with destgroups (see Setting hyperlink types and options),
     as it does not make sense in a direct call to
     ‘\hldest’--you can just call the raw command.

     Example:

          \makeatletter
          \def\mydest{\special{html:<a name="\@hllabel">}%
                      \special{html:</a>}}
          \resetatcatcode
          \indent\hldest{raw}{cmd=mydest}{SpecialDest}
            Special destination.

5.4.2.2 Destination options for ‘hypertex’
..........................................

‘cmd’
     Name of the macro (without the leading ‘\’) containing a HyperTeX
     ‘\special’ for the ‘raw’ destination.

     See Destination types for hypertex, the description of the
     ‘raw’ destination, for an example.

5.4.2.3 Link types for ‘hypertex’
.................................

‘name’
     Go to a "named destination". The label is the destination name. All
     destinations in HyperTeX are named destinations. This is the
     default type.

     Example: ‘\hlstart{name}{}{dest123}Link to dest123\hlend’

‘url’
     Go to a url. The label is the url.

     Example:

          \hlstart{url}{}{https://tug.org/eplain/}Eplain home\hlend

‘filename’
     Go to a named destination in another file. The label is the
     destination name. The file name is specified by the ‘file’ option.
     The file name extension can be specified separately by the ‘ext’
     option. The idea is to set the ‘ext’ option globally at the
     beginning of the document to avoid hard-coding the extension
     together with the file name within each link--HyperTeX is not
     restricted to any single output format, it can be DVI, PDF,
     possibly other formats.

     Example:

          \hlopts{ext=.pdf}
          \hlstart{filename}{file=book}{dest123}
            Link to dest123 in file `book.pdf'\hlend

‘raw’
     The link specification (in the form of a HyperTeX ‘\special’) is
     taken from the command sequence ‘\CS’, where CS is the value of the
     ‘cmd’ option. In the definition of ‘\CS’, use ‘\@hllabel’ to refer
     to the hyperlink label. Use the predefined command ‘\hlhash’ to
     insert the ‘#’ characters. This option is intended to be used with
     linkgroups (see Setting hyperlink types and options), as it
     does not make sense in a direct call to ‘\hlstart’--you can just
     call the raw command.

     Example:

          \makeatletter
          \def\mylink{\special{html:<a href="\hlhash\@hllabel">}}
          \resetatcatcode
          \hlstart{raw}{cmd=mylink}{SpecialDest}
            Link to the special destination.\hlend

5.4.2.4 Link options for ‘hypertex’
...................................

‘cmd’
     Name of the macro (without the leading ‘\’) containing a HyperTeX
     ‘\special’ for the ‘raw’ link.

     See Link types for hypertex, the description of the ‘raw’ link,
     for an example.

‘file’
     File name for the ‘filename’ link type. See also the ‘ext’ option.

     See Link types for hypertex, the description of the ‘filename’
     link, for an example.

‘ext’
     File name extension for the ‘filename’ link type. The idea is to
     set the ‘ext’ option globally at the beginning of the document to
     avoid hard-coding the extension together with the file name within
     each link--HyperTeX is not restricted to any single output format,
     it can be DVI, PDF, possibly other formats.

     See Link types for hypertex, the description of the ‘filename’
     link, for an example.

5.4.3 Hyperlink drivers ‘pdftex’ and ‘dvipdfm’
----------------------------------------------

This subsection describes link and destination types and options
supported by the ‘pdftex’ and ‘dvipdfm’ drivers. Many of the hyperlink
types and options are common to both drivers, so we describe them
together.

5.4.3.1 Destination types for ‘pdftex’ and ‘dvipdfm’
....................................................

‘xyz’
     "Current position". The option ‘zoom’ specifies magnification to
     use (zero or empty means leave magnification unchanged, which is
     the default). This is the default type.

     For ‘dvipdfm’: the options ‘left’ and ‘top’ specify position
     coordinates to use (empty options mean current position coordinate,
     which is the default).

     Example: ‘\hldest{xyz}{zoom=2000}{dest123}’

‘fit’
     Fit the page to the window.

     Example: ‘\hldest{fit}{}{dest123}’

‘fith’
     Fit the width of the page to the window.

     For ‘dvipdfm’: the ‘top’ option specifies vertical position
     (default is empty, meaning current position).

     Example: ‘\hldest{fith}{}{dest123}’

‘fitv’
     Fit the height of the page to the window.

     For ‘dvipdfm’: The option ‘left’ specifies horizontal position
     (default is empty, meaning current position).

     Example: ‘\hldest{fitv}{}{dest123}’

‘fitb’
     Fit the page's bounding box to the window.

     Example: ‘\hldest{fitb}{}{dest123}’

‘fitbh’
     Fit the width of the page's bounding box to the window.

     For ‘dvipdfm’: the option ‘top’ specifies vertical position
     (default is empty, meaning current position).

     Example: ‘\hldest{fitbh}{}{dest123}’

‘fitbv’
     Fit the height of the page's bounding box to the window.

     For ‘dvipdfm’: the option ‘left’ specifies horizontal position
     (default is empty, meaning current position).

     Example: ‘\hldest{fitbv}{}{dest123}’

‘fitr’
     For ‘pdftex’: fit the rectangle specified by the options ‘width’,
     ‘height’ and ‘depth’ (as a TeX rule specification) in the window.
     For dimensions set to empty, the corresponding value of the parent
     box is used (these are the defaults).

     For ‘dvipdfm’: fit the rectangle specified by the options ‘left’,
     ‘bottom’, ‘right’ and ‘top’ (in PostScript points, 72 points per
     inch) in the window. For dimensions set to empty, current position
     coordinate is substituted (these are the defaults).

     Example for ‘pdftex’:

          \hldest{fitr}{width=\hsize,
                        height=.5\vsize,depth=0pt}{dest123}

     Example for ‘dvipdfm’:

          \hldest{fitr}{left=72,bottom=72,
                        right=720,top=360}{dest123}

‘raw’
     The destination specification (in the form of a pdfTeX command or a
     dvipdfm ‘\special’) is taken from the command sequence ‘\CS’, where
     CS is the value of the ‘cmd’ option. In the definition of ‘\CS’,
     use ‘\@hllabel’ to refer to the hyperlink label. This option is
     intended to be used with destgroups (see Setting hyperlink types and options),
     as it does not make sense in a direct call to
     ‘\hldest’--you can just call the raw command.

     Example for ‘pdftex’:

          \makeatletter
          \def\mydest{\pdfdest name{\@hllabel} xyz}
          \resetatcatcode
          \hldesttype{raw}
          \hldestopts{cmd=mydest}

     Example for ‘dvipdfm’:

          \makeatletter
          \def\mydest{\special{pdf: dest (\@hllabel)
                                    [@thispage /XYZ @xpos @ypos 0]}}
          \resetatcatcode
          \hldesttype{raw}
          \hldestopts{cmd=mydest}

5.4.3.2 Destination options for ‘pdftex’ and ‘dvipdfm’
......................................................

With respect to the destination options, the ‘pdftex’ and ‘dvipdfm’
differ in the way the fit rectangle is specified (relative coordinates
for ‘pdftex’, absolute coordinates for ‘dvipdfm’).

Common destination options
..........................

‘cmd’
     Name of the macro (without the leading ‘\’) containing a pdfTeX
     command or a dvipdfm ‘\special’ for the ‘raw’ destination.

     See Destination types for pdftex and dvipdfm, the description
     of the ‘raw’ destination, for an example.

‘zoom’
     Magnification ratio times 1000 (like TeX's scale factor). Zero or
     empty means leave magnification unchanged, which is the default.

     Example: ‘\hldest{xyz}{zoom=2000}{dest123}’

‘pdftex’-specific destination options
.....................................

The dimension options below must be specified as a TeX rule
specification. When set to empty, the corresponding value of the parent
box is used (this is the default for all dimension options).

‘depth’
     Depth of the fit rectangle for the ‘fitr’ destination.

‘height’
     Height of the fit rectangle for the ‘fitr’ destination.

‘width’
     Width of the fit rectangle for the ‘fitr’ destination.

     Example:

          \hldest{fitr}{width=\hsize,
                        height=.5\vsize,depth=0pt}{dest123}

‘dvipdfm’-specific destination options
......................................

The dimension options below must be specified in PostScript points
(72 points per inch), as a number without the ‘bp’ unit name. When set
to empty, the current position coordinate is used (this is the default
for all dimension options).

‘bottom’
     Bottom position coordinate of a box specification for the various
     destination types.

‘left’
     Left position coordinate of a box specification for the various
     destination types.

‘right’
     Right position coordinate of a box specification for the various
     destination types.

‘top’
     Top position coordinate of a box specification for the various
     destination types.

     Example:

          \hldest{fitr}{left=72,bottom=72,
                        right=720,top=360}{dest123}

5.4.3.3 Link types for ‘pdftex’ and ‘dvipdfm’
.............................................

Link types are the same for the ‘pdftex’ and ‘dvipdfm’ drivers, except
that the ‘pdftex’ driver provides one additional link type ‘num’ (link
to a numbered destination). dvipdfm does not support numbered
destinations, therefore it does not have this link type. Note that all
destinations created by Eplain hyperlink macros are named destinations;
to define a numbered destination, you have to use low-level pdfTeX
commands.

Common link types
.................

‘name’
     Go to a "named destination". The label is the destination name. All
     destinations created with ‘\hldest’ are named destinations (see
     Explicit hyperlinks). This is the default type.

     Example: ‘\hlstart{name}{}{dest123}Link to dest123\hlend’

‘url’
     Go to a url. The label is the url.

     Example: ‘\hlstart{url}{}{https://tug.org/eplain/}Eplain
     home\hlend’

‘page’
     Go to a page. The label is the page number (counting from 1). Page
     fitting is specified by the ‘pagefit’ option.

     Example:

          \hlstart{page}{pagefit=/FitH 600}{123}
            Link to page~123\hlend

‘filename’
     Go to a named destination in another file. The label is the
     destination name. The file name is specified by the ‘file’ option.
     Page fitting is specified by the ‘pagefit’ option. The ‘newwin’
     option specifies whether the destination document is opened in the
     same window or in a new window.

     Example:

          \hlstart{filename}{file=book.pdf,newwin=1}{dest123}
            Link to dest123 in file `book.pdf'\hlend

‘filepage’
     Go to a page in another file. The label is the page number
     (counting from 1). The file name is specified by the ‘file’ option.
     Page fitting is specified by the ‘pagefit’ option. The ‘newwin’
     option specifies whether the destination document is opened in the
     same window or in a new window.

     Example:

          \hlstart{filepage}{file=book.pdf,newwin=,
                             pagefit=/FitR 50 100 300 500}{1}
            Link to page~1 in file `book.pdf'\hlend

‘raw’
     The link specification (in the form of a pdfTeX command or a
     dvipdfm ‘\special’ primitive) is taken from the command sequence
     ‘\CS’, where CS is the value of the ‘cmd’ option. In the definition
     of ‘\CS’, use ‘\@hllabel’ to refer to the hyperlink label. This
     option is intended to be used with linkgroups (see Setting hyperlink types and options),
     as it does not make sense in a
     direct call to ‘\hlstart’--you can just call the raw command.

     Example for ‘pdftex’:

          % Redirect all \url links to the first page
          \def\mycmd{\pdfstartlink goto page 1 {/Fit}}
          \hltype[url]{raw}
          \hlopts[url]{cmd=mycmd}

     Example for ‘dvipdfm’:

          % Redirect all \url links to the first page
          \def\mycmd{\special{pdf: beginann <</Type/Annot /Subtype/Link
                                              /Dest[0 /Fit]>>}}
          \hltype[url]{raw}
          \hlopts[url]{cmd=mycmd}

‘pdftex’-specific link types
............................

‘num’
     Go to a "numbered destination". The label is the destination
     number.

     Example: ‘\hlstart{num}{}{123}Link to 123\hlend’

5.4.3.4 Link options for ‘pdftex’ and ‘dvipdfm’
...............................................

Link options are mostly the same for the ‘pdftex’ and ‘dvipdfm’ drivers.
The ‘pdftex’ driver has additional options to specify link dimensions.

Common link options
...................

‘bcolor’
     Border color. An array of three numbers in the range 0 to 1,
     representing a color in DeviceRGB.

     Example: ‘\hlstart{name}{bcolor=.1 .5 1}{dest123}Link\hlend’

‘bdash’
     Array of numbers representing on and off stroke lengths for drawing
     dashes.

     Example: ‘\hlstart{name}{bstyle=D,bdash=2 4}{dest123}Link\hlend’

‘bstyle’
     Link border style:

     ‘S’
          The border is drawn as a solid line.

     ‘D’
          The border is drawn with a dashed line (the dash pattern is
          specified by the ‘bdash’ option).

     ‘B’
          The border is drawn in a beveled style.

     ‘I’
          The border is drawn in an inset style.

     ‘U’
          The border is drawn as a line on the bottom of the link
          rectangle.

     The default is ‘S’.

     Example: ‘\hlstart{name}{bstyle=D,bdash=2 4}{dest123}Link\hlend’

‘bwidth’
     Border width in PostScript points (72 points per inch). The default
     is 1.

     Example: ‘\hlstart{name}{bwidth=2}{dest123}Link\hlend’

‘cmd’
     Name of the macro (without the leading ‘\’) containing a pdfTeX
     command or a dvipdfm ‘\special’ for the ‘raw’ link.

     See Link types for pdftex and dvipdfm, the description of the
     ‘raw’ link, for an example.

‘file’
     File name for the ‘filename’ and ‘filepage’ link types.

     See Link types for pdftex and dvipdfm, the descriptions of the
     ‘filename’ and ‘filepage’ links, for an example.

‘hlight’
     Link border highlight modes:

     ‘I’
          The rectangle specified by the bounding box of the link is
          inverted.

     ‘N’
          No highlighting is done.

     ‘O’
          The border of the link is inverted.

     ‘P’
          The region underneath the bounding box of the link is drawn
          inset into the page.

     The default is ‘I’.

     Example: ‘\hlstart{name}{bstyle=S,hlight=O}{dest123}Link\hlend’

‘newwin’
     For the ‘filename’ and ‘filepage’ links, specifies whether the
     destination document is opened in the same window or in a new
     window. The settings are:

     ‘0’
          Open in the same window.
     ‘non-0’
          Open in a new window.
     ‘empty’
          Behavior according to the viewer settings.

     The default is empty.

     See Link types for pdftex and dvipdfm, the descriptions of the
     ‘filename’ and ‘filepage’ links, for an example.

‘pagefit’
     For the ‘page’ and ‘filepage links’, specifies how the page must be
     fitted to the window. ‘pagefit’ specification is written to the PDF
     file as is, so it must conform to the PDF standard.

     See Link types for pdftex and dvipdfm, the descriptions of the
     ‘page’ and ‘filepage’ links, for an example.

‘pdftex’-specific link options
..............................

The dimension options below must be specified as a TeX rule
specification. When set to empty, the corresponding value of the parent
box is used (this is the default for all dimension options).

‘depth’
     Depth of the link.

‘height’
     Height of the link.

‘width’
     Width of the link.

     Example:

          \hlstart{name}{width=5in,height=20pc,depth=0pt}{dest123}
            Link\hlend

5.4.4 Hyperlink driver ‘nolinks’
--------------------------------

Select this driver to suppress all hyperlinks in your document.

   Selecting this driver is quite different from not selecting any
driver at all, or from selecting some driver and then turning hyperlinks
off for the entire document with ‘\hloff’ and ‘\hldestoff’ (see
Turning hyperlinks on/off).

   The purpose of ‘\hldestoff’ and ‘\hloff’ is to mark (parts) of your
document where hyperlinks should never appear. (Imagine you want to
prevent a cross-referencing macro from generating a link at a certain
spot in your document.)

   If instead you have prepared a document with hyperlinks and just want
to compile a version without them, it is better to select the driver
‘nolinks’. This ensures that spacing and page-breaking are the same as
what you were getting with hyperlinks enabled.

   The reason for this is that hyperlinks are produced by the ‘\special’
primitives or low-level hyperlink commands. Each such command is placed
inside a “whatsit” (an internal TeX object), which may introduce
legitimate breakpoints at places where none would exist without the
whatsits. The macros ‘\hldestoff’ and ‘\hloff’ disable the hyperlink
macros completely, so that no whatsits are produced.

   In contrast, the ‘nolinks’ driver does not completely disable
hyperlink macros. Instead, it defines them to merely write to the log
file (what gets written is unimportant). This also produces whatsits,
thus imitating the whatsits from the hyperlink commands. (This trick was
borrowed from the LaTeX 'color' package.)

   Another reason for using ‘nolinks’ is that in horizontal mode
‘\hldest’ places destinations inside boxes of zero width, height, and
depth. When you say ‘\hldestoff’, ‘\hldest’ will omit both destination
definitions and these boxes. The missing boxes can again cause the
typesetting to be inconsistent with that with destinations enabled. Here
again, the ‘nolinks’ driver helps by defining ‘\hldest’ to produce the
empty boxes.

   So, if you are planning to produce versions of your PDF document both
with and without hyperlinks, here is the recommended way to enable the
hyperlinks under pdfTeX:

     \ifpdf
       \enablehyperlinks
     \else
       \enablehyperlinks[nolinks]%
     \fi

5.5 Setting hyperlink types and options
=======================================

You can define default types for links and destinations, which will be
used when you do not specify a type in ‘\hlstart’ or ‘\hldest’.
Similarly, you can define default values for the options; the default
value for an option is used when you do not set the option in the
argument to ‘\hlstart’ or ‘\hldest’.

   The parameters for implicit links and destinations can be customized
by setting the "group" types and options. When not set, the defaults are
used.

   All these settings are local to the current (TeX) group, so if you
want to set an option temporarily, you can do so inside a
‘\begingroup...\endgroup’ block; when the group ends, the previous
settings are restored.

5.5.1 Setting default types and options
---------------------------------------

The default types for links and destinations can be set with the
following commands:

     \hltype{TYPE}
     \hldesttype{TYPE}

where TYPE is one of the link/destination types supported by the
selected hyperlink driver (see Hyperlink drivers).

   Default values for the options can be set with the following
commands:

     \hlopts{OPTIONS}
     \hldestopts{OPTIONS}

where OPTIONS is a comma-separated list of option assignments in the
format ‘OPTION=VALUE’. Again, what options are allowed depends on the
selected hyperlink driver.

   Many people regard the default boxed links as not aesthetic and
intruding on page layout. The reason why boxed links are the default is
that the links are not colored until you load the LaTeX 'color' package
(see Loading LaTeX packages) or use other means to define the
‘\color’ command; therefore, not producing any kind of link border may
result in the links not being marked in any way. However, when the links
are actually colored, there is no need to produce the link boxes
anymore; to avoid the boxes, you can set the default border width to
zero (if the driver you use supports the link option ‘bwidth’; see
Hyperlink drivers):

     \hlopts{bwidth=0}

5.5.2 Setting group types
-------------------------

When called with an optional argument, as in

     \hltype[GROUPS]{TYPE}
     \hldesttype[GROUPS]{TYPE}

where GROUPS is a comma-separated list of groups, ‘\hltype’ and
‘\hldesttype’ set the type for each group from GROUPS to TYPE. The
default type is used for all groups with an empty type (this is the
initial setting for all groups, except that the type for the 'url'
linkgroup is set to ‘url’ by the drivers which support this link type).

   There are two special "groups" which can be used inside the GROUPS
list. An empty group sets the default type. This allows to set the
default type and group types in one command, for example:

     \hltype[eq,]{TYPE}

sets the link type for the 'eq' linkgroup and the default link type to
TYPE.

   Another special group is a star (‘*’) group, which signifies all
defined groups. For example, the command

     \hldesttype[*,]{TYPE}

sets the destination type to TYPE for each group, as well as the default
destination type.

5.5.3 Setting group options
---------------------------

Option values for each group are stored as a list of option assignments.
This list does not have to contain every supported option. Values for
options missing from this list are taken from the default option values.

   To manipulate the list of option values for the groups, you use the
‘\hlopts’ and ‘\hldestopts’ commands with an optional argument:

     \hlopts[GROUPS]{OPTIONS}
     \hldestopts[GROUPS]{OPTIONS}
     \hlopts![GROUPS]{OPTIONS}
     \hldestopts![GROUPS]{OPTIONS}

where GROUPS is a comma-separated list of groups and OPTIONS is a
comma-separated list of option assignments. The two special "groups",
the empty group and the star (‘*’) group, have the same meaning as for
‘\hltype’ and ‘\hldesttype’. When used without the exclamation mark,
‘\hlopts’ and ‘\hldestopts’ preserve the current list of options for the
groups, and only update the options listed in OPTIONS. If you add the
exclamation mark, the current list of options for each group in GROUPS
is discarded and the new list is set to OPTIONS.

   The "overriding" nature of the ‘!’ is appropriate when you give a
complete specification of the options for a group, e.g., at the
beginning of your document. On the other hand, when you want to adjust
some option(s) and leave others intact, you should use the macros
without the ‘!’.

   For example, with displayed mathematical formulas, you often need to
adjust the ‘raise’ option for the 'eq' destgroup, because the formulas
often contain large parentheses and brackets. But when doing so, you
want to leave the other settings unchanged. To achieve this, call
‘\hldestopts’ without the ‘!’, for example:

     $$\hldestopts[eq]{raise=2.5\normalbaselineskip}
     ...
     $$

The display commands (‘$$’) implicitly put the entire formula inside a
(TeX) group (‘\begingroup...\endgroup’), so you do not need to isolate
the setting of the ‘raise’ option--it will be restored after the closing
‘$$’.

   Initially, Eplain sets the option lists for almost all groups to
empty, so that the groups fall back on the default values for all
options. The one exception to this rule is the 'eq' destgroup, whose
initial option list contains one setting:

     raise=1.7\normalbaselineskip

This setting usually accommodates the large operators, which often
appear in displayed math.

5.6 Turning hyperlinks on/off
=============================

Links and/or destinations can be turned on or off globally by disabling
the low-level commands, or for each group individually.

   All these settings are local to the current (TeX) group, so if you
want to enable or disable links/destinations temporarily, you can do so
inside a ‘\begingroup...\endgroup’ block; when the group ends, the
previous settings are restored.

5.6.1 Turning low-level commands on/off
---------------------------------------

The low-level commands ‘\hlstart’, ‘\hlend’ and ‘\hldest’ can be turned
on/off with the following commands:

     \hldeston
     \hldestoff
     \hlon
     \hloff

See Hyperlink driver nolinks, for the implications of using these
commands to disable hyperlinks for the entire document.

5.6.2 Turning hyperlinks on/off for a group
-------------------------------------------

If you want to disable links/destinations produced by certain groups,
you can give a list of the groups as an optional argument to these
commands:

     \hldeston[GROUPS]
     \hldestoff[GROUPS]
     \hlon[GROUPS]
     \hloff[GROUPS]

where GROUPS is the list of linkgroups/destgroups. This list can contain
two special groups. The empty group switches the low-level commands
(see Turning low-level commands on/off), and the star (‘*’) group
operates on all defined groups.

   Note that turning off the low-level commands disables all hyperlinks
globally, including groups which have them enabled. Turning off certain
linkgroups/destgroups records the fact that the macros in the group
should not produce links/destinations. To illustrate the distinction,
assume that all links are on; after the following sequence of commands:

     \hloff
     \hloff[eq]
     \hlon

all links are on except for the 'eq' linkgroup.

5.7 Making PDF outlines
=======================

PDF outlines (a.k.a. bookmarks) are more or less a table of contents
that PDF viewers can display alongside the main document. Eplain's
hyperlink features can be used to create them; there isn't any special
support for them. A continuing example interspersed with commentary
follows.

   First we must enable hyperlinks.
     \input eplain
     \enablehyperlinks %[dvipdfm] doesn't work

   We will separate the code to support ‘pdftex’ from ‘dvips’ with the
‘\ifpdf’ conditional (provided by Eplain).

   For ‘pdftex’, we can use the ‘\pdfoutline’ primitive. The keyword
"count" is followed by the number of subentries in this entry. If
negative, the bookmark is closed (that is, subentries are hidden).

     \ifpdf
       \pdfoutline goto name {sec1} count -1 {Mysec-pdf}%
       \pdfoutline goto name {sec1.1} {Mysubsec-pdf}%

   For ‘dvips’, we use TeX's ‘\special’ command to emit a ‘ps:’ special
using the PDF ‘pdfmark’ operator. The ‘ps:’ prefix tells ‘dvips’ that
the following is literal PostScript.

   ‘[ ... pdfmark’ (there is no closing ‘]’) is a extension to the
PostScript language for specifying various PDF-related things. It is
recognized by Ghostscript, Distiller, et al. Adobe publishes a reference
manual for it:
<https://adobe.com/content/dam/Adobe/en/devnet/acrobat/pdfs/pdfmark_reference.pdf>.

   The ‘/DOCVIEW’ pdfmark used here says the outline panel should be
used.

     \else % not pdf output
       \special{ps:[/PageMode /UseOutlines /DOCVIEW pdfmark}
       %
       % The individual outline entries, using a different syntax
       % than pdftex, but the same information.
       \special{ps:[/Count -1 /Dest (sec1) cvn /Title (Mysec-dvi)
                     /OUT pdfmark}
       \special{ps:[/Count -0 /Dest (sec1.1) cvn /Title (Mysubsec-dvi)
                     /OUT pdfmark}
     \fi

   The ‘-pdf’ and ‘-dvi’ suffixes in the strings above in the outline
entries are just to make it clear which branch is being executed, for
purposes of this example. Ordinarily the entries would be the same in
both branches.

   Also, the strings above are literal PostScript constants, again for
this example. Usually they would come from control sequences, e.g., as
the table of contents is read.

   It is necessary to "pdf-escape" such arbitrary strings, else
backslashes, parentheses, etc., would not come out right. pdfTeX's
‘\pdfescapestring’ primitive is an easy way to do this, e.g.,
‘\xdef#1{\pdfescapestring{#1}}’.

   Here is the document text, constructing three pages with the section
and subsection given above in the outlines.

     First page.\vfil\eject

     \hldest{}{}{sec1}%
     1. Mysec on second page.\vfil\eject

     \hldest{}{}{sec1.1}%
     1.1. Mysubsec on third page.

     \end

6 Arrow theoretic diagrams
**************************

This chapter describes definitions for producing commutative diagrams.

   Steven Smith wrote this documentation (and the macros).

6.1 Slanted lines and vectors
=============================

The macros ‘\drawline’ and ‘\drawvector’ provide the capability found in
LaTeX's picture mode to draw slanted lines and vectors of certain
directions. Both of these macros take three arguments: two integer
arguments to specify the direction of the line or vector, and one
argument to specify its length. For example, ‘\drawvector(-4,1){60pt}’
produces the vector

      (A vector in the 2d quadrant of length 60 pt appears here.)

   which lies in the 2d quadrant, has a slope of minus 1/4, and a width
of 60 pt.

   Note that if an ‘\hbox’ is placed around ‘\drawline’ or
‘\drawvector’, then the width of the ‘\hbox’ will be the positive
dimension specified in the third argument, except when a vertical line
or vector is specified, e.g., ‘\drawline(0,1){1in}’, which has zero
width. If the specified direction lies in the 1st or 2d quadrant (e.g.,
‘(1,1)’ or ‘(-2,3)’), then the ‘\hbox’ will have positive height and
zero depth. Conversely, if the specified direction lies in the 3d or 4th
quadrant (e.g., ‘(-1,-1)’ or ‘(2,-3)’), then the ‘\hbox’ will have
positive depth and zero height.

   There are a finite number of directions that can be specified. For
‘\drawline’, the absolute value of each integer defining the direction
must be less than or equal to six, i.e., ‘(7,-1)’ is incorrect, but
‘(6,-1)’ is acceptable. For ‘\drawvector’, the absolute value of each
integer must be less than or equal to four. Furthermore, the two
integers cannot have common divisors; therefore, if a line with slope 2
is desired, say ‘(2,1)’ instead of ‘(4,2)’. Also, specify ‘(1,0)’
instead of, say, ‘(3,0)’ for horizontal lines and likewise for vertical
lines.

   Finally, these macros depend upon the LaTeX font ‘line10’. If your
site doesn't have this font, ask your system administrator to get it.
Future enhancements will include macros to draw dotted lines and dotted
vectors of various directions.

6.2 Commutative diagrams
========================

The primitive commands ‘\drawline’ and ‘\drawvector’ can be used to
typeset arrow theoretic diagrams. This section describes (1) macros to
facilitate typesetting arrows and morphisms, and (2) macros to
facilitate the construction of commutative diagrams. All macros
described in this section must be used in math mode.

6.2.1 Arrows and morphisms
--------------------------

The macros ‘\mapright’ and ‘\mapleft’ produce right and left pointing
arrows, respectively. Use superscript (‘^’) to place a morphism above
the arrow, e.g., ‘\mapright^\alpha’; use subscript (‘_’) to place a
morphism below the arrow, e.g., ‘\mapright_{\tilde l}’. Superscripts and
subscripts may be used simultaneously, e.g., ‘\mapright^\pi_{\rm
epimor.}’.

   Similarly, the macros ‘\mapup’ and ‘\mapdown’ produce up and down
pointing arrows, respectively. Use ‘\rt’ to place a morphism to the
right of the arrow, e.g., ‘\mapup\rt{\rm id}’; use ‘\lft’ to place a
morphism to the left of the arrow, e.g., ‘\mapup\lft\omega’. ‘\lft’ and
‘\rt’ may be used simultaneously, e.g., ‘\mapdown\lft\pi\rt{\rm
monomor.}’.

   Slanted arrows are produced by the macro ‘\arrow’, which takes a
direction argument (e.g., ‘\arrow(3,-4)’). Use ‘\rt’ and ‘\lft’ to place
morphisms to the right and left, respectively, of the arrow. A slanted
line (no arrowhead) is produced with the macro ‘\sline’, whose syntax is
identical to that of ‘\arrow’.

   The length of these macros is predefined by the default TeX
dimensions ‘\harrowlength’, for horizontal arrows (or lines),
‘\varrowlength’, for vertical arrows (or lines), and ‘\sarrowlength’,
for slanted arrows (or lines). To change any of these dimensions, say,
e.g., ‘\harrowlength=40pt’. As with all other TeX dimensions, the change
may be as global or as local as you like. Furthermore, the placement of
morphisms on the arrows is controlled by the dimensions ‘\hmorphposn’,
‘\vmorphposn’, and ‘\morphdist’. The first two dimensions control the
horizontal and vertical position of the morphism from its default
position; the latter dimension controls the distance of the morphism
from the arrow. If you have more than one morphism per arrow (i.e., a
‘^’/‘_’ or ‘\lft’/‘\rt’ construction), use the parameters
‘\hmorphposnup’, ‘\hmorphposndn’, ‘\vmorphposnup’, ‘\vmorphposndn’,
‘\hmorphposnrt’, ‘\hmorphposnlft’, ‘\vmorphposnrt’, and
‘\vmorphposnlft’. The default values of all these dimensions are
provided in the section on parameters that follows below.

   There is a family of macros to produce horizontal lines, arrows, and
adjoint arrows. The following macros produce horizontal maps and have
the same syntax as ‘\mapright’:

‘\mapright’
     ‘$X\mapright Y$’ = (a right arrow).

‘\mapleft’
     ‘$X\mapleft Y$’ = (a left arrow).

‘\hline’
     ‘$X\hline Y$’ = (horizontal line)

‘\bimapright’
     ‘$X\bimapright Y$’ = (two right arrows).

‘\bimapleft’
     ‘$X\bimapleft Y$’ = (two left arrows)

‘\adjmapright’
     ‘$X\adjmapright Y$’ = (two adjoint arrows; left over right)

‘\adjmapleft’
     ‘$X\adjmapleft Y$’ = (two adjoint arrows; right over left)

‘\bihline’
     ‘$X\bihline Y$’ = (two horizontal lines)

   There is also a family of macros to produce vertical lines, arrows,
and adjoint arrows. The following macros produce vertical maps and have
the same syntax as ‘\mapdown’:

‘\mapdown’
     (a down arrow)

‘\mapup’
     (an up arrow)

‘\vline’
     (vertical line)

‘\bimapdown’
     (two down arrows)

‘\bimapup’
     (two up arrows)

‘\adjmapdown’
     (two adjoint arrows; down then up)

‘\adjmapup’
     (two adjoint arrows; up then down)

‘\bivline’
     (two vertical lines)

   Finally, there is a family of macros to produce slanted lines,
arrows, and adjoint arrows. The following macros produce slanted maps
and have the same syntax as ‘\arrow’:

‘\arrow’
     (a slanted arrow)

‘\sline’
     (a slanted line)

‘\biarrow’
     (two straight arrows)

‘\adjarrow’
     (two adjoint arrows)

‘\bisline’
     (two straight lines)

   The width between double arrows is controlled by the parameter
‘\channelwidth’. The parameters ‘\hchannel’ and ‘\vchannel’, if nonzero,
override ‘\channelwidth’ by controlling the horizontal and vertical
shifting from the first arrow to the second.

   There are no adornments on these arrows to distinguish inclusions
from epimorphisms from monomorphisms. Many texts, such as Lang's book
‘Algebra’, use as a tasteful alternative the symbol 'inc' (in roman)
next to an arrow to denote inclusion.

   Future enhancements will include a mechanism to draw curved arrows
found in, e.g., the Snake Lemma, by employing a version of the ‘\path’
macros of Appendix D of ‘The TeXbook’.

6.2.2 Construction of commutative diagrams
------------------------------------------

There are two approaches to the construction of commutative diagrams
described here. The first approach, and the simplest, treats commutative
diagrams like fancy matrices, as Knuth does in Exercise 18.46 of ‘The
TeXbook’. This case is covered by the macro ‘\commdiag’, which is an
altered version of the Plain TeX macro ‘\matrix’. An example suffices to
demonstrate this macro. The following commutative diagram (illustrating
the covering homotopy property; Bott and Tu, ‘Differential Forms in
Algebraic Topology’)

      (A commutative diagram appears here in the printed output.)

   is produced with the code

     $$\commdiag{Y&\mapright^f&E\cr \mapdown&\arrow(3,2)\lft{f_t}&\mapdown\cr
     Y\times I&\mapright^{\bar f_t}&X}$$

   Of course, the parameters may be changed to produce a different
effect. The following commutative diagram (illustrating the universal
mapping property; Warner, ‘Foundations of Differentiable Manifolds and
Lie Groups’)

      (A commutative diagram appears here in the printed output.)

   is produced with the code

     $$\varrowlength=20pt
     \commdiag{V\otimes W\cr \mapup\lft\phi&\arrow(3,-1)\rt{\tilde l}\cr
     V\times W&\mapright^l&U\cr}$$

   A diagram containing isosceles triangles is achieved by placing the
apex of the triangle in the center column, as shown in the example
(illustrating all constant minimal realizations of a linear system;
Brockett, ‘Finite Dimensional Linear Systems’)

      (A commutative diagram appears here in the printed output.)

   which is produced with the code

     $$\sarrowlength=.42\harrowlength
     \commdiag{&R^m\cr &\arrow(-1,-1)\lft{\bf B}\quad \arrow(1,-1)\rt{\bf G}\cr
     R^n&\mapright^{\bf P}&R^n\cr
     \mapdown\lft{e^{{\bf A}t}}&&\mapdown\rt{e^{{\bf F}t}}\cr
     R^n&\mapright^{\bf P}&R^n\cr
     &\arrow(1,-1)\lft{\bf C}\quad \arrow(-1,-1)\rt{\bf H}\cr
     &R^q\cr}$$

   Other commutative diagram examples appear in the file
‘commdiags.tex’, which is distributed with this package.

   In these examples the arrow lengths and line slopes were carefully
chosen to blend with each other. In the first example, the default
settings for the arrow lengths are used, but a direction for the arrow
must be chosen. The ratio of the default horizontal and vertical arrow
lengths is approximately the golden mean gamma=1.618...; the arrow
direction closest to this mean is ‘(3,2)’. In the second example, a
slope of -1/3 is desired and the default horizontal arrow length is 60
pt; therefore, choose a vertical arrow length of 20 pt. You may affect
the interline glue settings of ‘\commdiag’ by redefining the macro
‘\commdiagbaselines’. (cf. Exercise 18.46 of ‘The TeXbook’ and the
section on parameters below.)

   The width, height, and depth of all morphisms are hidden so that the
morphisms' size do not affect arrow positions. This can cause a large
morphism at the top or bottom of a diagram to impinge upon the text
surrounding the diagram. To overcome this problem, use TeX's ‘\noalign’
primitive to insert a ‘\vskip’ immediately above or below the offending
line, e.g., ‘$$\commdiag{\noalign{\vskip6pt}X&\mapright^\int&Y\cr ...}’.

   The macro ‘\commdiag’ is too simple to be used for more complicated
diagrams, which may have intersecting or overlapping arrows. A second
approach, borrowed from Francis Borceux's ‘Diagram’ macros for LaTeX,
treats the commutative diagram like a grid of identically shaped boxes.
To compose the commutative diagram, first draw an equally spaced grid,
e.g.,

                              . . . . . .
                              . . . . . .
                              . . . . . .
                              . . . . . .

   on a piece of scratch paper. Then draw each element (vertices and
arrows) of the commutative diagram on this grid, centered at each grid
point. Finally, use the macro ‘\gridcommdiag’ to implement your design
as a TeX alignment. For example, the cubic diagram

                 (A commutative diagram appears here.)

   that appears in Francis Borceux's documentation can be implemented on
a 7 by 7 grid, and is achieved with the code

     $$\harrowlength=48pt \varrowlength=48pt \sarrowlength=20pt
     \def\cross#1#2{\setbox0=\hbox{$#1$}%
       \hbox to\wd0{\hss\hbox{$#2$}\hss}\llap{\unhbox0}}
     \gridcommdiag{&&B&&\mapright^b&&D\cr
     &\arrow(1,1)\lft a&&&&\arrow(1,1)\lft d\cr
     A&&\cross{\hmorphposn=12pt\mapright^c}{\vmorphposn=-12pt\mapdown\lft f}
     &&C&&\mapdown\rt h\cr\cr
     \mapdown\lft e&&F&&\cross{\hmorphposn=-12pt\mapright_j}
     {\vmorphposn=12pt\mapdown\rt g}&&H\cr
     &\arrow(1,1)\lft i&&&&\arrow(1,1)\rt l\cr
     E&&\mapright_k&&G\cr}$$

   The dimensions ‘\hgrid’ and ‘\vgrid’ control the horizontal and
vertical spacing of the grid used by ‘\gridcommdiag’. The default
setting for both of these dimensions is 15 pt. Note that in the example
of the cube the arrow lengths must be adjusted so that the arrows
overlap into neighboring boxes by the desired amount. Hence, the
‘\gridcommdiag’ method, albeit more powerful, is less automatic than the
simpler ‘\commdiag’ method. Furthermore, the ad hoc macro ‘\cross’ is
introduced to allow the effect of overlapping arrows. Finally, note that
the positions of four of the morphisms are adjusted by setting
‘\hmorphposn’ and ‘\vmorphposn’.

   One is not restricted to a square grid. For example, the proof of
Zassenhaus's Butterfly Lemma can be illustrated by the diagram
(appearing in Lang's book ‘Algebra’)

                 (A commutative diagram appears here.)

   This diagram may be implemented on a 9 by 12 grid with an aspect
ratio of 1/2, and is set with the code

     $$\hgrid=16pt \vgrid=8pt \sarrowlength=32pt
     \def\cross#1#2{\setbox0=\hbox{$#1$}%
       \hbox to\wd0{\hss\hbox{$#2$}\hss}\llap{\unhbox0}}
     \def\l#1{\llap{$#1$\hskip.5em}}
     \def\r#1{\rlap{\hskip.5em$#1$}}
     \gridcommdiag{&&U&&&&V\cr &&\bullet&&&&\bullet\cr
     &&\sarrowlength=16pt\sline(0,1)&&&&\sarrowlength=16pt\sline(0,1)\cr
     &&\l{u(U\cap V)}\bullet&&&&\bullet\r{(U\cap V)v}\cr
     &&&\sline(2,-1)&&\sline(2,1)\cr
     &&\cross{=}{\sline(0,1)}&&\bullet&&\cross{=}{\sline(0,1)}\cr\cr
     &&\l{^{\textstyle u(U\cap v)}}\bullet&&\cross{=}{\sline(0,1)}&&
      \bullet\r{^{\textstyle(u\cap V)v}}\cr
     &\sline(2,1)&&\sline(2,-1)&&\sline(2,1)&&\sline(2,-1)\cr
     \l{u}\bullet&&&&\bullet&&&&\bullet\r{v}\cr
     &\sline(2,-1)&&\sline(2,1)&&\sline(2,-1)&&\sline(2,1)\cr
     &&\bullet&&&&\bullet\cr &&u\cap V&&&&U\cap v\cr}$$

   Again, the construction of this diagram requires careful choices for
the arrow lengths and is facilitated by the introduction of the ad hoc
macros ‘\cross’, ‘\r’, and ‘\l’. Note also that superscripts were used
to adjust the position of the vertices u(U intersection v) and (u
intersection V)v. Many diagrams may be typeset with the predefined
macros that appear here; however, ingenuity is often required to handle
special cases.

6.2.3 Commutative diagram parameters
------------------------------------

The following is a list describing the parameters used in the
commutative diagram macros. These dimensions may be changed globally or
locally.

‘\harrowlength’
     (Default: 60 pt) The length of right or left arrows.

‘\varrowlength’
     (Default: 0.618‘\harrowlength’) The length of up or down arrows.

‘\sarrowlength’
     (Default: 60 pt) The horizontal length of slanted arrows.

‘\hmorphposn’
     (Default: 0 pt) The horizontal position of the morphism with
     respect to its default position. There are also the dimensions
     ‘\hmorphposnup’, ‘\hmorphposndn’, ‘\hmorphposnrt’, and
     ‘\hmorphposnlft’ for ‘^’/‘_’ or ‘\lft’/‘\rt’ constructions.

‘\vmorphposn’
     (Default: 0 pt) The vertical position of the morphism with respect
     to its default position. There are also the dimensions
     ‘\vmorphposnup’, ‘\vmorphposndn’, ‘\vmorphposnrt’, and
     ‘\vmorphposnlft’ for ‘^’/‘_’ or ‘\lft’/‘\rt’ constructions.

‘\morphdist’
     (Default: 4 pt) The distance of morphisms from slanted lines or
     arrows.

‘\channelwidth’
     (Default: 3 pt) The distance between double lines or arrows.

‘\hchannel, \vchannel’
     (Defaults: 0 pt) Overrides ‘\channelwidth’. The horizontal and
     vertical shifts between double lines or arrows.

‘\commdiagbaselines’
     (Default: ‘\baselineskip=15pt \lineskip=3pt \lineskiplimit=3pt ’)
     The parameters used by ‘\commdiag’ for setting interline glue.

‘\hgrid’
     (Default: 15 pt) The horizontal spacing of the grid used by
     ‘\gridcommdiag’.

‘\vgrid’
     (Default: 15 pt) The vertical spacing of the grid used by
     ‘\gridcommdiag’.

7 Programming definitions
*************************

The definitions in this section are only likely to be useful when you
are writing nontrivial macros, not when writing a document.

7.1 Category codes
==================

Plain TeX defines ‘\active’ (as the number 13) for use in changing
category codes. Although the author of ‘The TeXbook’ has "intentionally
kept the category codes numeric", two other categories are commonly
used: letters (category code 11) and others (12). Therefore, Eplain
defines ‘\letter’ and ‘\other’.

   Sometimes it is cleaner to make a character active without actually
writing a ‘\catcode’ command. The ‘\makeactive’ command takes a
character as an argument to make active (and ignores following spaces).
For example, here are two commands which both make ‘\’ active:

     \makeactive\\   \makeactive92

   Sometimes you might want to temporarily change the category code of
the '@' character to ‘\letter’, so that you can use or define macros
which are normally inaccessible to the user. For such situations, Eplain
provides the ‘\makeatletter’ command. It sets the category code of '@'
to ‘\letter’ (11) and defines ‘\resetatcatcode’ to restore the category
code to whatever it was before the call to ‘\makeatletter’. For example:

     \makeatletter
     \def\@hidden@macro{This macro cannot normally be
                        called / redefined by the user}
     \resetatcatcode

There is also ‘\makeatother’ which works similarly but sets the category
code of '@' to ‘\other’ (12).

   Usually, when you give a definition to an active character, you have
to do so inside a group where you temporarily make the character active,
and then give it a global definition (cf. the definition of
‘\obeyspaces’ in ‘The TeXbook’). This is inconvenient if you are writing
a long macro, or if the character already has a global definition you do
not wish to transcend. Eplain provides ‘\letreturn’, which defines the
usual end-of-line character to be the argument. For example:

     \def\mymacro{... \letreturn\myreturn ... }
     \mymacro hello
     there

The end-of-line between ‘hello’ and ‘there’ causes ‘\myreturn’ to be
expanded.

   ‘The TeXbook’ describes ‘\uncatcodespecials’, which makes all
characters which are normally "special" into "other" characters, but the
definition never made it into plain TeX. Eplain therefore defines it.
For notes on the usage, see Verbatim listing.

   Finally, ‘\percentchar’ expands into a literal '%' character. This is
useful when you ‘\write’ TeX output to a file, and want to avoid
spurious spaces. For example, Eplain writes a ‘\percentchar’ after the
definition of cross-references. The macros ‘\lbracechar’ and
‘\rbracechar’ expand similarly.

7.2 Allocation macros
=====================

Plain TeX provides macros that allocate registers of each primitive type
in TeX, to prevent different sets of macros from using the same register
for two different things. The macros are all named starting with ‘new’,
e.g., ‘\newcount’ allocates a new "count" (integer) register. Such
allocations are usually needed only at the top level of some macro
definition file; therefore, plain TeX makes the allocation registers
‘\outer’, to help find errors. (The error this helps to find is a
missing right brace in some macro definition.)

   Sometimes, however, it is useful to allocate a register as part of
some macro. An outer control sequence cannot be used as part of a macro
definition (or in a few other contexts: the parameter text of a
definition, an argument to a definition, the preamble of an alignment,
or in conditional text that is being skipped). Therefore, Eplain defines
"inner" versions of all the allocation macros, named with the prefix
‘inner’: ‘\innernewbox’, ‘\innernewcount’, ‘\innernewdimen’,
‘\innernewfam’, ‘\innernewhelp’, ‘\innernewif’, ‘\innernewinsert’,
‘\innernewlanguage’, ‘\innernewread’,
‘\innernewskip’, ‘\innernewtoks’, ‘\innernewwrite’.

   You can also define non-outer versions of other macros in the same
way that Eplain defines the above. The basic macro is called
‘\innerdef’:

     \innerdef \INNERNAME {OUTERNAME}

   The first argument (\INNERNAME) to ‘\innerdef’ is the control
sequence that you want to define. Any previous definition of \INNERNAME
is replaced. The second argument (OUTERNAME) is the _characters_ in the
name of the outer control sequence. (You can't use the actual control
sequence name, since it's outer!)

   If the outer control sequence is named \CS, and you want to define
‘innerCS’ as the inner one, you can use ‘\innerinnerdef’, which is just
an abbreviation for a call to ‘\innerdef’. For example, these two calls
are equivalent:

     \innerdef\innerproclaim{proclaim}
     \innerinnerdef{proclaim}

7.2.1 Scratch registers
-----------------------

It seems worth reiterating here the conventions for scratch registers
defined by Knuth for plain TeX, which Eplain inherits and does not
change.

   • Registers numbered 0 to 9, and 255, are free for any use. Thus
     their values must be assumed to be clobbered whenever another macro
     might get control. Eplain macros do use the scratch registers.

   • First exception: the ‘\count’ registers ‘0..9’ are used internally
     by TeX for page numbering and thus are not available for any other
     use.

   • Second exception: the ‘\box255’ register is likewise internally by
     TeX, to ship out pages. So it's not available either.

   • By convention, all assignments to the scratch registers 1, 3, 5, 7,
     and 9 should be ‘\global’, and assignments to the others should be
     non-‘\global’.

   For more details on all aspects of register usage, see ‘The TeXbook’
or any other plain TeX reference.

7.3 Iteration
=============

You can iterate through a comma-separated list of items with ‘\for’.
Here is an example:

     \for\name:=karl,kathy\do{%
        \message{\name}%
     }%

This writes ‘karl’ and ‘kathy’ to the terminal. Spaces before or after
the commas in the list, or after the ‘:=’, are _not_ ignored. To strip
leading spaces off the items, use ‘\For’:

     \For\name:=
        karl,
        kathy\do{%
           \message{\name}%
        }%

Note that trailing spaces are still _not_ ignored.

   Both ‘\for’ and ‘\For’ expand the first token of the item list fully,
so this is equivalent to the above:

     \def\namelist{karl,kathy}%
     \for\name:=\namelist\do ...

However, this won't work, either with ‘\for’ or with ‘\For’:

     \def\namelist{karl,kathy}%
     \For\name:= \namelist\do ...

because ‘\for’ and ‘\For’ expand the first token after ‘:=’ which is
space, not ‘\namelist’.

   Eplain provides another kind of loops, which is an extension of plain
TeX's ‘\loop’. If you say:

     \loop
       LOOP-TEXT
     \if CONDITION
       IF-TEXT
     \repeat

then LOOP-TEXT will be repeated as long as CONDITION is satisfied (‘\if’
can be any of the TeX's conditional commands, without the matching
‘\fi’). Eplain extends this with the optional else clause:

     \loop
       LOOP-TEXT
     \if CONDITION
       IF-TEXT
     \else
       ELSE-TEXT
     \repeat

Here, LOOP-TEXT will be repeated as long as CONDITION is _not_
satisfied. This extension is from Victor Eijkhout's TeX by Topic
(page 104).

7.4 Macro arguments
===================

It is occasionally useful to redefine a macro that takes arguments to do
nothing. Eplain defines ‘\gobble’, ‘\gobbletwo’, and ‘\gobblethree’ to
swallow one, two, and three arguments, respectively.

   For example, if you want to produce a "short" table of contents--one
that includes only chapters, say--the easiest thing to do is read the
entire ‘.toc’ file (see Contents), and just ignore the commands that
produce section or subsection entries. To be specific:

     \let\tocchapterentry = \shorttocchapter
     \let\tocsectionentry = \gobbletwo
     \let\tocsubsectionentry = \gobbletwo
     \readtocfile

(Of course, this assumes you only have chapters, sections, and
subsections in your document.)

   In addition, Eplain defines ‘\eattoken’ to swallow the single
following token, using ‘\let’. Thus, ‘\gobble’ followed by ‘{...}’
ignores the entire brace-enclosed text. ‘\eattoken’ followed by the same
ignores only the opening left brace.

   Eplain defines a macro ‘\identity’ which takes one argument and
expands to that argument. This may be useful if you want to provide a
function for the user to redefine, but don't need to do anything by
default. (For example, the default definition of ‘\eqconstruct’ (see
Formatting equation references) is ‘\identity’.)

   You may also want to read an optional argument. The established
convention is that optional arguments are put in square brackets, so
that is the syntax Eplain recognizes. Eplain ignores space tokens before
and after optional arguments, via ‘\futurenonspacelet’.

   You test for an optional argument by using ‘\@getoptionalarg’. It
takes one argument, a control sequence to expand after reading the
argument, if present. If an optional argument is present, the control
sequence ‘\@optionalarg’ expands to it; otherwise, ‘\@optionalarg’ is
‘\empty’. You must therefore have the category code of ‘@’ set to 11
(letter). Here is an example:

     \catcode`@=\letter
     \def\cmd{\@getoptionalarg\finishcmd}
     \def\finishcmd{%
       \ifx\@optionalarg\empty
         % No optional argument present.
       \else
         % One was present.
       \fi
     }

   It's possible to define macros that appear to accept optional
arguments intermixed with mandatory arguments in any imaginable way. For
example:

     \makeatletter
     % \mo{m}[o]
     \def\mo#1{\def\mo@arg{#1}\@getoptionalarg\fin@mo}
     \def\fin@mo{\vskip1pc
       ARG:    \mo@arg       \par
       OPTARG: \@optionalarg \par
     }
     % \mom{m}[o]{m}
     \def\mom#1{\def\mom@arg{#1}\@getoptionalarg\fin@mom}
     \def\fin@mom#1{\vskip1pc
       ARG1:   \mom@arg      \par
       OPTARG: \@optionalarg \par
       ARG2:   #1\par
     }
     % \omo[o]{m}[o]
     \def\omo{\@getoptionalarg\fin@omo}
     \def\fin@omo#1{\let\omo@optarg\@optionalarg
                    \def\omo@arg{#1}\@getoptionalarg\@fin@omo}
     \def\@fin@omo{\vskip1pc
       OPTARG1: \omo@optarg   \par
       ARG:     \omo@arg      \par
       OPTARG2: \@optionalarg \par
     }
     \makeatother

   If an optional argument contains another optional argument, the inner
one will need to be enclosed in braces, so TeX does not mistake the end
of the first for the end of the second.

7.5 Converting to characters
============================

Eplain defines ‘\xrlabel’ to produce control sequence names for
cross-reference labels, et al. This macro expands to its argument with
an ‘_’ appended. (It does this because the usual use of ‘\xrlabel’ is to
generate a control sequence name, and we naturally want to avoid
conflicts between control sequence names.)

   Because ‘\xrlabel’ is fully expandable, to make a control sequence
name out of the result you need only do

     \csname \xrlabel{LABEL}\endcsname

The ‘\csname’ primitive makes a control sequence name out of any
sequence of character tokens, regardless of category code. Labels can
therefore include any characters except for ‘\’, ‘{’, ‘}’, and ‘#’, all
of which are used in macro definitions themselves.

   ‘\sanitize’ takes a control sequence as an argument and converts the
expansion of the control sequence into a list of character tokens. This
is the behavior you want when writing information like chapter titles to
an output file. For example, here is part of the definition of
‘\writenumberedtocentry’; ‘#2’ is the title that the user has given.

     ...
     \def\temp{#2}%
     ...
       \write\tocfile{%
         ...
         \sanitize\temp
         ...
       }%

7.6 Expansion
=============

This section describes some miscellanous macros for expansion, etc.

7.6.1 ‘\csn’ and ‘\ece’
-----------------------

‘\csn’{NAME} simply abbreviates ‘\csname’ NAME ‘\endcsname’, thus saving
some typing. The extra level of expansion does take some time, though,
so I don't recommend it for an inner loop.

   ‘\ece’{TOKEN}{NAME} abbreviates

     \expandafter TOKEN \csname NAME \endcsname

For example,

     \def\fontabbrevdef#1#2{\ece\def{@#1font}{#2}}
     \fontabbrevdef{normal}{ptmr}

defines a control sequence ‘\@normalfont’ to expand to ‘ptmr’.

7.6.2 ‘\edefappend’
-------------------

‘\edefappend’ is a way of adding on to an existing definition. It takes
two arguments: the first is the control sequence name, the second the
new tokens to append to the definition. The second argument is fully
expanded (in the ‘\edef’ that redefines the control sequence).

   For example:

     \def\foo{abc}
     \def\bar{xyz}
     \edefappend\foo{\bar karl}

results in ‘\foo’ being defined as ‘abcxyzkarl’.

7.6.3 Hooks
-----------

A “hook” is simply a name for a group of actions which is executed in
certain places--presumably when it is most useful to allow customization
or modification. TeX already provides many builtin hooks; for example,
the ‘\every ...’ token lists are all examples of hooks.

   Eplain provides several macros for adding actions to hooks. They all
take two arguments: the name of the hook and the new actions.

‘hookaction NAME ACTIONS’
‘hookappend NAME ACTIONS’
‘hookprepend NAME ACTIONS’
     Each of these adds ACTIONS to the hook NAME. (Any
     previously-defined actions are retained.) NAME is not a control
     sequence, but rather the characters of the name.

‘hookactiononce NAME \CS’
     ‘\hookactiononce’ adds CS to NAME, like the macros above, but first
     it adds

          \global\let \CS \relax

     to the definition of \CS. (This implies \CS must be a true
     expandable macro, not a control sequence ‘\let’ to a primitive or
     some other such thing.) Thus, \CS is expanded the next time the
     hook NAME is run, but it will disappear after that.

     The ‘\global’ is useful because ‘\hookactiononce’ is most useful
     when the grouping structure of the TeX code could be anything.
     Neither this nor the other hook macros do global assignments to the
     hook variable itself, so TeX's usual grouping rules apply.

   The companion macro to defining hook actions is ‘\hookrun’, for
running them. This takes a single argument, the name of the hook. If no
actions for the hook are defined, no error ensues.

   Here is a skeleton of general ‘\begin’ and ‘\end’ macros that run
hooks, and a couple of calls to define actions. The use of
‘\hookprepend’ for the begin action and ‘\hookappend’ for the end action
ensures that the actions are executed in proper sequence with other
actions (as long as the other actions use ‘\hookprepend’ and
‘\hookappend’ also).

     \def\begin#1{ ... \hookrun{begin} ... }
     \def\end#1{ ... \hookrun{end} ... }
     \hookprepend{begin}\start_underline
     \hookappend{end}\finish_underline

7.6.4 Properties
----------------

A “property” is a name/value pair associated with another symbol,
traditionally called an “atom”. Both atom and property names are control
sequence names.

   Eplain provides two macros for dealing with property lists:
‘\setproperty’ and ‘\getproperty’.

‘\setproperty ATOM PROPNAME VALUE’
     ‘\setproperty’ defines the property PROPERTY on the atom ATOM to be
     VALUE. ATOM and PROPNAME can be anything acceptable to ‘\csname’.
     VALUE can be anything.

‘\getproperty ATOM PROPNAME’
     ‘\getproperty’ expands to the value stored for PROPNAME on ATOM. If
     PROPNAME is undefined, it expands to nothing (i.e., ‘\empty’).

   The idea of properties originated in Lisp (I believe). There, the
implementation truly does associate properties with atoms. In TeX, where
we have no builtin support for properties, the association is only
conceptual.

   The following example typesets ‘xyz’.

     \setproperty{a}{pr}{xyz}
     \getproperty{a}{pr}

7.6.5 ‘\expandonce’
-------------------

‘\expandonce’ is defined as ‘\expandafter\noexpand’. Thus, ‘\expandonce
TOKEN’ expands TOKEN once, instead of to TeX primitives. This is most
useful in an ‘\edef’.

   For example, the following defines ‘\temp’ to be ‘\foo’, not ‘abc’.

     \def\foo{abc}
     \def\bar{\foo}
     \edef\temp{\expandonce\bar}

7.6.6 ‘\ifundefined’
--------------------

‘\ifundefined{CS} T \else F \fi’ expands the T text if the control
sequence ‘\CS’ is undefined or has been ‘\let’ to ‘\relax’, and the F
text otherwise.

   Since ‘\ifundefined’ is not a primitive conditional, it cannot be
used in places where TeX might skip tokens "at high speed", e.g., within
another conditional--TeX can't match up the ‘\if’'s and ‘\fi’'s.

   This macro was taken directly from ‘The TeXbook’, page 308.

7.6.7 ‘\ifempty’
----------------

‘\ifempty{ARG} T \else F \fi’ expands the T text if ARG is the empty
string, and the F text otherwise. This macro is useful when you need to
test for empty arguments to your macros, for example:

     \def\foo#1{\ifempty{#1} T \else F \fi}

   Since ‘\ifempty’ is not a primitive conditional, it cannot be used in
places where TeX might skip tokens "at high speed", e.g., within another
conditional--TeX can't match up the ‘\if’'s and ‘\fi’'s.

   Note that the following code

     \def\empty{}
     \ifempty\empty\message{empty}\else\message{not empty}\fi

will produce the message ‘not empty’.

7.6.8 ‘\ifinteger’ and ‘\isinteger’
-----------------------------------

‘\ifinteger{ARG} T \else F \fi’ expands the T text if ARG is an integer,
and the F text otherwise. This macro can detect positive and negative
integers.

   Since ‘\ifinteger’ is not a primitive conditional, it cannot be used
in places where TeX might skip tokens "at high speed", e.g., within
another conditional--TeX can't match up the ‘\if’'s and ‘\fi’'s. For
such situations Eplain provides ‘\isinteger’, which can be used as
follows:

     \if\isinteger{ARG} T \else F \fi

   Although ‘\ifinteger’ and ‘\isinteger’ work well with regular input,
they are not bullet-proof. For example, the following code

     \ifinteger{12_ab}integer\else not integer\fi

will expand to ‘ab_integer’ (and thus would not even compile outside
math mode).

   These macros come from the TeX Frequently Asked Questions
(<https://texfaq.org>).

7.6.9 ‘\futurenonspacelet’
--------------------------

The ‘\futurelet’ primitive allows you to look at the next token from the
input. Sometimes, though, you want to look ahead while ignoring any
spaces. This is what ‘\futurenonspacelet’ does. It is otherwise the same
as ‘\futurelet’: you give it two control sequences as arguments, and it
assigns the next nonspace token to the first, and then expands the
second. For example:

     \futurenonspacelet\temp\finishup
     \def\finishup{\ifx\temp ...}

7.7 Obeying spaces
==================

‘\obeywhitespace’ makes both end-of-lines and space characters in the
input be respected in the output. Unlike plain TeX's ‘\obeyspaces’, even
spaces at the beginnings of lines turn into blank space.

   By default, the size of the space that is produced by a space
character is the natural space of the current font, i.e., what ‘\ ’
produces.

   Ordinarily, a blank line in the input produces as much blank vertical
space as a line of text would occupy. You can adjust this by assigning
to the parameter ‘\blanklineskipamount’: if you set this negative, the
space produced by a blank line will be smaller; if positive, larger.

   Tabs are not affected by this routine. In particular, if tabs occur
at the beginning of a line, they will disappear. (If you are trying to
make TeX do the "right thing" with tabs, don't. Use a utility program
like expand instead.)

7.8 Writing out numbers
=======================

‘\numbername’ produces the written-out form of its argument, i.e.,
'zero' through 'ten' for the numbers 0-10, and numerals for all others.

7.9 Mode-specific penalties
===========================

TeX's built-in ‘\penalty’ command simply appends to the current list, no
matter what kind of list it is. You might intend a particular penalty to
always be a "vertical" penalty, however, i.e., appended to a vertical
list. Therefore, Eplain provides ‘\vpenalty’ and ‘\hpenalty’ which first
leave the other mode and then do ‘\penalty’.

   More precisely, ‘\vpenalty’ inserts ‘\par’ if the current mode is
horizontal, and ‘\hpenalty’ inserts ‘\leavevmode’ if the current mode is
vertical. (Thus, ‘\vpenalty’ cannot be used in math mode.)

7.10 Auxiliary files
====================

It is common to write some information out to a file to be used on a
subsequent run. But when it is time to read the file again, you only
want to do so if the file actually exists. ‘\testfileexistence’ is given
an argument which is appended to ‘\jobname’, and sets the conditional
‘\iffileexists’ appropriately. For example:

     \testfileexistence{toc}%
     \iffileexists
        \input \jobname.toc
     \fi

   ‘\testfileexistence’ takes an optional parameter; when given, it will
override ‘\jobname’ for the root part of the file name. For example, if
you want to test for the file ‘answers.aux’, you can do this with the
following:

     \testfileexistence[answers]{aux}%
     \iffileexists
        \input answers.aux
     \fi

7.11 User-defined environments
==============================

Plain TeX does not provide "named" block structures, only the anonymous
‘\begingroup’ and ‘\endgroup’ pair. The disadvantage of this is that
when there are several such groups and one is mismatched, it can be
difficult to find the error. Eplain provides a named block structure so
that if you forget an ‘\environment’ or an ‘\endenvironment’, you will
(probably) get an error message about it.

   For example:

     \def\itpar{
       \environment{@italicpar}
       \it\par
     }
     \def\enditpar{
       \par
       \endenvironment{@italicpar}%
     }

which could then be used to set italicized paragraphs:

     \itpar
     If I reprehend anything in this world, it is the use of my oracular
     tongue, and a nice derangement of epitaphs!
     \enditpar

   The above sort of environment allows nesting. But environments
shouldn't always be allowed to nest. Put the control sequence
‘\checkenv’ at the beginning of a macro that is going to define an
environment that should not be nested.

7.12 Page list and page range parsers
=====================================

The macros which Eplain uses to parse the page lists and ranges in the
index, ‘\idxparselist’ and ‘\idxparserange’ (see Page destinations for index terms),
are sometimes useful when defining page number
encapsulators. They take one argument, text to parse. When a page list
(range) is not present, they set ‘\idxpagei’ to be ‘\empty’; when a list
(range) is detected, they set ‘\idxpagei’ and ‘\idxpageii’ to the first
and the second page numbers, respectively.

   Eplain's defaults for the page list and page range delimiters are the
same as those in MakeIndex, a comma followed by a space (‘, ’) and two
dashes (‘--’), respectively. If you customize MakeIndex to use different
delimiters, you must not forget to let Eplain know about them with the
commands

     \setidxpagelistdelimiter{LIST-DELIM}
     \setidxpagerangedelimiter{PAGE-DELIM}

These commands save the LIST-DELIM and PAGE-DELIM delimiters in
‘\idxpagelistdelimiter’ and ‘\idxpagerangedelimiter’, respectively.

   For example, you may want to define a page number markup command
which italicizes and properly underlines page ranges by underlining only
the page numbers and not the delimiter:

     \def\ituline#1{%
       {\it
       \idxparserange{#1}%
       \ifx\idxpagei\empty
         % The argument is a single page number.
         \underbar{#1}%
       \else
         % The argument is a page range.
         \underbar{\idxpagei}\idxpagerangedelimiter\underbar{\idxpageii}%
       \fi}%
     }

Note that the ‘\ituline’ macro is not aware of page lists. This is not
needed if you use hyperlinks in the index, because ‘\hlidx’ and
‘\hlidxpage’ will break up the page lists before calling the user's page
encapsulator (see Page destinations for index terms), so ‘\ituline’
will never see the lists. If, however, you need to design a macro which
also takes care of the lists, you can extend ‘\ituline’ with an
additional call to ‘\idxparselist’.

8 Demo files
************

This chapter contains listings of source files, along with the output
they produce (where appropriate), which illustrate various aspects of
Eplain. The files can be found in the ‘demo’ subdirectory of Eplain
distribution. These demos, both the sources and the compiled PDF and PS
files, are also available from <https://tug.org/eplain/demo>.

8.1 Hyperlinks (‘xhyper.tex’)
=============================

% $Id: xhyper.tex 60 2022-10-05 22:42:54Z karl $
% (This file is public domain.)
%
% This file demonstrates the following features of Eplain:
%
%   - explicit and implicit hyperlinks;
%   - symbolic cross-references;
%   - inserting external graphics using |\includegraphics| from
%     the \LaTeX\ package |graphicx.sty|.
%   - rotating text using |\rotatebox| from |graphicx.sty|.
%
% The compiled demo can be downloaded from
%
%   https://tug.org/eplain/demo
%
% In order to compile this file yourself, you will need the CTAN lion
% drawing by Duane Bibby from
%
%   ftp://tug.ctan.org/ftpmaint/CTAN_lion/ctan_lion_350x350.png
%
% (thanks, |www.ctan.org|).  Place the image file in the same
% directory with this file, and change to that directory.  Now, to
% produce a PDF, run twice the command
%
%   pdftex xhyper.tex
%
% During the first run, Eplain will write the information about the
% cross-references into |xhyper.aux|, and during the second run this
% information will be read by Eplain to typeset the references.
%
% Demo case:
%
%   Suppose you are using pdf\TeX, have a figure you want to insert
%   scaled to $200\,pt$ horizontally, and you want this figure to
%   completely fill the PDF viewer's window whenever the reader
%   selects a link pointing to this figure.  Additionally, you want to
%   typeset some text as live hyperlinks, clicking on which will start
%   a Web browser and open a URL.

\input eplain

% Load \LaTeX\ packages.
\beginpackages
  % |url.sty| provides the |\url| command which we will use to typeset
  % a URL.  We request that |url.sty| be the version from June~27,
  % 2005, or later, because earlier versions had problems interacting
  % with plain \TeX.
  \usepackage{url}[2005/06/27]
  % |color.sty| provides support for colored text; all hyperlinks are
  % automatically colored by Eplain when this package is loaded.  We give
  % the |dvipsnames| option because we want to use named colors from the
  % |dvips| graphics driver.
  \usepackage[dvipsnames]{color}
  % Finally, we load |graphicx.sty|, for the macros |\includegraphics|
  % and |\rotatebox|.
  \usepackage{graphicx}
\endpackages

% Remember that hyperlinks are off by default.  Therefore, we need to
% enable them.
\enablehyperlinks

% Customize some hyperlink options.  First, we set border width to~$0$
% for all links to get rid of the default boxes around links (we
% prefer colored links over the boxed links). Next, we say that all
% links created by the |url| hyperlink group (which means the |\url|
% command from |url.sty|) must be colored using the named color
% |BlueViolet|.
\hlopts{bwidth=0}
\hlopts[url]{colormodel=named,color=BlueViolet}

% Inhibit page numbering (we have only one page).
\nopagenumbers

% Define a class word for the cross-reference class |figure|.  This
% word, when defined, will be automatically prepended by Eplain to the
% reference created by |\ref| (read on to see its use).
\def\figureword{fig.}

% Allocate a count register for the figure's number, and a box
% register which we'll use to measure dimensions of the image.
\newcount\fignumber
\newbox\imgbox

% Now comes the fun part--constructing the figure for the image of the
% \CTAN\ lion.  We define a macro
%
%   \fig{LABEL}{FILENAME}{CAPTION}{WIDTH}
%
% which creates a figure using LABEL for the cross-reference and
% hyperlink label, reading the image from file FILENAME, using CAPTION
% to name the figure, and WIDTH to scale the image horizontally.
\def\fig#1#2#3#4{%
  % Leave some space above the figure.  This will also ensure that we
  % are in the vertical mode before we produce a |\vbox|.
  \medskip
  % Advance the figure number.  |\global| ensures that the change to
  % the count register is global, even when |\fig| is buried inside a
  % group.
  \global\advance\fignumber by 1
  % We use |\includegraphics| (from |graphicx.sty|) to load the image,
  % scaled to the specified width, into our ``measuring'' box
  % |\imgbox|.
  \setbox\imgbox = \hbox{\includegraphics[width=#4]{#2}}%
  % To make the demo even more exciting, let's put the figure's
  % caption to the left of the image into the |\indent| space of the
  % new paragraph, and rotate the caption~$90^\circ$.
  \textindent{%
    \rotatebox{90}{F{\sc IGURE}~\the\fignumber.  #3}%
  }%
  % Continue the paragraph by constructing a |\vbox| with the image of
  % the lion.  We use |\definexref| to define the cross-reference
  % label.
  \vbox{%
    % In addition to the cross-reference label, |\definexref| will
    % create a hyperlink destination with the same label.  Therefore,
    % we customize this destination to do what we need.  We say that
    % destination type for the hyperlink group |definexref| (to which
    % |\definexref| belongs) should be |fitr|.  This destination type
    % will magnify the rectangle specified by the options |width|,
    % |height| and |depth| to the PDF viewer's window.  Therefore, we
    % set those options accordingly with |\hldestopts| (notice the use
    % of |depth| instead of |height|---we will want the rectangle to
    % extend {\it downward}, to cover the image which will come
    % next).  Notice that these settings will be isolated within the
    % current group (i.e., the |\vbox| we're constructing).
    \hldesttype[definexref]{fitr}%
    \hldestopts[definexref]{width=\wd\imgbox,height=0pt,depth=\ht\imgbox}%
    % We define a symbolic label so that we can later refer
    % to the figure with |\ref|.  The command |\definexref| does
    % exactly that.  The last argument to |\definexref| specifies
    % class of the label, which determines the word used by |\ref| in
    % front of the reference text (remember that we've defined
    % |\figureword| above).
    \definexref{#1}{\the\fignumber}{figure}%
    % Finally, produce the image which we've been stashing in the box
    % register |\imgbox|.
    \box\imgbox
  }%
  \medskip
}

% Create the figure.
\fig{CTANlion}{ctan_lion_350x350}{Lion in the archives}{200pt}

% Finished with the fun part, we can relax and typeset some
% hyperlinks.  The easiest way to do that is to use the |\ref|
% cross-reference command.  We can even pass an optional argument
% (|the lion in|), which will be placed before the class word (|fig.|)
% and become part of the link (to make sure the reader does not have
% to aim too hard).
Show me \ref[the lion in]{CTANlion}.

% If you are the restless kind, here's another way to create a
% hyperlink to the image:  we create a link explicitly by using
% |\hlstart ... \hlend|.  We don't specify the link type, therefore
% the default type |name| will be used (these are ``named links'',
% i.e., links pointing to destinations identified by labels).  In the
% options argument, we specify that the border of the link should be
% inverted when the user clicks the link (|hlight=O|), and also set
% special color for this link, overriding the default dark-red.  The
% label for the destination is the same as the cross-reference label,
% |CTANlion|.
Show me
\hlstart{}{hlight=O,colormodel=named,color=OliveGreen}{CTANlion}
  the CTAN lion\hlend.

% Let's now point somewhere outside our document.  Eplain homepage is
% a good target.  In the similar spirit, let's consider two
% approaches.  The easy one is to use the |\url| command from
% |url.sty|.  Remember that we have customized the color of |url|
% hyperlinks, so this one will show up with a different color than the
% default dark-red.
Take me to \url{https://tug.org/eplain}.

% The second approach is to create an explicit URL link.  We specify
% yet another border highlighting mode, |P|, which means that the
% region underneath the bounding box of the link will be drawn inset
% into the page.  Also, let's set the color of the hyperlink to an RGB
% color |0.4,0.1,0.4|.  Since we cannot use commas to separate the
% color elements inside the options parameter to |\hlstart| (commas
% there separate options), we have to first create a user-defined
% color with |\definecolor| from |color.sty|, and use that in
% |\hlstart|.
\definecolor{mycolor}{rgb}{0.4,0.1,0.4}

Take me to
\hlstart{url}{hlight=P,colormodel=,color=mycolor}{https://tug.org/eplain}
  Eplain homepage\hlend.

\bye

8.2 Highlighting TeX comments in listings (‘lscommnt.tex’)
==========================================================

% (This file is public domain.)
% Demonstrate how Eplain can be used to include a TeX source file
% verbatim, typesetting comments in colored italic typewriter type.

% Load Eplain and LaTeX's color.sty package.
\input eplain
\beginpackages \usepackage{color} \endpackages
\nopagenumbers % Disable page numbers.
\font\commentfont = cmitt10 % Font for the comments (italic \tt).
% We'll define some `protected' macros with `@' in their names.
\makeatletter
% Define an equivalent of Eplain's \letreturn, to be able to assign
% various actions to the (active) percent character.
\begingroup \makeactive\%
  \gdef\letpercent{\let%}
\endgroup
% The listing hook to be called in \setuplistinghook, see below.  It
% makes `%' active and assigns it a `start comment' action.
\def\hlightcommentslisting{\makeactive\% \letpercent\start@comment}%
% This is what `%' is aliased to before a comment is started.
\def\start@comment{%
  \leavevmode % Needed in the very first line of the input to process
              % the new par (possibly inserting line number) before we
              % kick in with the color and stuff.
  \begingroup % Isolate color and font changes and the redefinitions.
    \commentfont
    \color[cmyk]{0.28,1,1,0.35}%
    \percentchar            % Produce the actual `%' and
    \letpercent\percentchar % make all following `%'s do the same.
    \letreturn\end@comment  % Call \end@comment at end-of-line.
}
% \end@comment (alias for ^^M inside a comment) will end the comment
% started by \start@comment.  We make ^^M active temporarily so that
% the active version of ^^M gets "frozen" into \end@comment.
\begingroup \makeactive\^^M % Avoid ^^M's from here on.
  \gdef\end@comment{\endgroup ^^M}%
\endgroup
\resetatcatcode % Make `@' again inaccessible for use in macro names.

% Define \setuplistinghook to setup comments highlighting, line
% numbering and omitting the last (empty) line of input.
\def\setuplistinghook{\hlightcommentslisting \linenumberedlisting
                      \nolastlinelisting}
% Isn't this fun?  This file typesets itself, with the extra bonus of
% the pretty-printed comments and numbered source lines!
\listing{lscommnt}
\bye