%% 
%%  The LaTeX Graphics Companion, 2ed (first printing May 2007)
%% 
%%  Example 7-4-21 on page 493.
%% 
%%  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
\[
\xymatrix @=15mm {
  L_1 \rlowertwocell<-3>_{a_1}{<-1>}
      \rcompositemap<6>_{a_2}^{a_3}{\omit}
& R_1 \dtwocell<0>_{a_4}{"}
\\ L_2 \uuppertwocell_u^d{\omit}
       \rtwocell<2>{'id}
&  R_2
 }\]
\end{document}