%% 
%%  The LaTeX Graphics Companion, 2ed (first printing May 2007)
%% 
%%  Example 7-2-7 on page 472.
%% 
%%  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[frame,arrow,curve]{xy}
\StopShownPreambleCommands
\begin{document}
\[\begin{xy}
  (0,0)*@{*}*[F]{Box1}!UL="a",*@{x}
 ,(20,0)*@{*}*[F]{Box2}!RD*@{x}
 ,\ar@/^20pt/"a", \POS"a"!DR(.8)*@{+}
\end{xy}\]
\end{document}
