\documentclass{ppex}
\nonstopmode
\usepackage [all,web]{xy}\usepackage {ifthen}
\pagestyle{empty}
\setlength\textwidth{183.0pt}
\begin{document}
\ResetPreambleCommands
\ReadyForTheFray
\usepackage [all,web]{xy}\usepackage {ifthen}
\[\renewcommand{\latticebody}{%
  \ifthenelse{\latticeA=1}{%
    \ifthenelse{\latticeB=1}{\drop{\dir{*}}}%
                            {\drop{\dir{o}}}}%
    {\drop{\dir{x}}}}
\begin{xy}
 *\xybox{0;<8mm,4mm>:<4mm,8mm>::
 ,0,{\croplattice{-6}{6}{-6}{6}%
                 {-3}{3}{-3.5}{3.5}}%
     }="S"
 ,{"S"+L \ar "S"+R*+!L{z_1}}
 ,{"S"+D \ar "S"+U*+!D{z_2}}
\end{xy}\]
\end{document}
