%% 
%%  The LaTeX Graphics Companion, 2ed (first printing May 2007)
%% 
%%  Example 7-4-23 on page 495.
%% 
%%  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[all,2cell]{xy}
\StopShownPreambleCommands
\begin{document}
\UseTwocells
\[
\xymatrix @R=2.0pc @C=.8pc {
& \bullet \ar @/^1ex/ [rr]
&& \bullet \ar @/^1ex/ [dr]
\\ \bullet \urtwocell<2> \rrtwocell<2>
           \drtwocell<2> \xtwocell[1,3]{}\omit
           \xtwocell[-1,3]{}\omit
&& \bullet \ar @/_1ex/ [ur]
           \ar @/^1ex/ [dr]
           \rrtwocell\omit
&& \bullet
\\ & \bullet \ar @/_1ex/ [rr]
&&   \bullet \ar @/_1ex/ [ur]
}\]
\end{document}