%% 
%%  The LaTeX Graphics Companion, 2ed (first printing May 2007)
%% 
%%  Example 7-4-34 on page 502.
%% 
%%  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,web]{xy}
\StopShownPreambleCommands
\begin{document}
\[\renewcommand{\latticebody}{\drop@{o}}
\begin{xy}
 *\xybox{0;<5mm,2mm>:<2mm,5mm>::
 ,0,{\xylattice{-3}{3}{-3}{3}}}="S"
 ,{"S"+L \ar "S"+R*+!L{z_1}}
 ,{"S"+D \ar "S"+U*+!D{z_2}}
\end{xy}\]

\end{document}