%% 
%%  The LaTeX Graphics Companion, 2ed (first printing May 2007)
%% 
%%  Example 7-4-6 on page 483.
%% 
%%  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{159.83385pt}
\StartShownPreambleCommands
\usepackage{diagxy}
\StopShownPreambleCommands
\begin{document}
\[\bfig
 \square/@3{->}`~)`=o`--x/[X_1`X_2`X_3`X_4;1`2`3`4]
 \place(400,100)[\twoar(-1,-1)]
 \place(100,400)[\twoar(1,1)]
 \morphism(500,500)||/{*}.{*}/<-500,-500>[X_2`X_3;]
\efig\]
\end{document}