%% 
%%  The LaTeX Graphics Companion, 2ed (first printing May 2007)
%% 
%%  Example 7-4-22 on page 494.
%% 
%%  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{207.83385pt}
\StartShownPreambleCommands
\usepackage[all,2cell]{xy}
\StopShownPreambleCommands
\begin{document}
\UseAllTwocells
\[
\renewcommand{\objectstyle}{\scriptstyle}
\renewcommand{\labelstyle}{\scriptstyle}
\xymatrix @=1pc {
&& \bullet \ar[2,2]
\\ \\
   \bullet \ar[-2,2]
        \rrtwocell {=}
        \xtwocell[0,4]{}\omit{^<-4>a}
&&  x_b \ar[rr] \ar[dr]_{\mathrm{id}_x}
        \rrtwocell\omit{<1.5>}
&& \bullet
\\ &&& x_e \ar[ur]
}\]
\end{document}