\documentclass{ppex}
\nonstopmode
\usepackage [all,web]{xy}
\pagestyle{empty}
\setlength\textwidth{183.0pt}
\begin{document}
\ResetPreambleCommands
\ReadyForTheFray
\usepackage [all,web]{xy}
\[\renewcommand{\latticebody}{\drop@{o}}
\begin{xy}
 *\xybox{0;<5mm,2mm>:<2mm,5mm>::
 ,0,{\xylattice{-3}{3}{-3}{3}}}="S"
 ,{"S"+L \ar "S"+R*+!L{z_1}}
 ,{"S"+D \ar "S"+U*+!D{z_2}}
\end{xy}\]

\end{document}