%% 
%%  The LaTeX Graphics Companion, 2ed (first printing May 2007)
%% 
%%  Example 7-2-2 on page 470.
%% 
%%  Copyright (C) 2007 Michel Goossens, Frank Mittelbach, Denis Roegel, Sebastian Rahtz, Herbert Vo\ss 
%% 
%%  It may be distributed and/or modified under the conditions
%%  of the LaTeX Project Public License, either version 1.3
%%  of this license or (at your option) any later version.
%% 
%%  See http://www.latex-project.org/lppl.txt for details.
%% 

\documentclass{ttctexa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{183.83385pt}
\StartShownPreambleCommands
\usepackage{xy}
\StopShownPreambleCommands
\begin{document}
\[\begin{xy} 0;<5mm,0mm>:
 0*{DL} ,(2,0)*{DR} ,<0cm,1cm>*{UL} ,(2,2)*{UR}
 ,0+(4,0)="NO"*{\mathtt{DL}}
 ,"NO"+(2,2)="SUR"*{\mathtt{UR}}
 ,"SUR"-<1cm,0cm>*{\mathtt{UL}}
 ,"SUR"-(0,2)*{\mathtt{DR}}
\end{xy}\]
\end{document}
