\documentclass{ppex}
\nonstopmode
\usepackage {xy}
\pagestyle{empty}
\setlength\textwidth{183.0pt}
\begin{document}
\ResetPreambleCommands
\ReadyForTheFray
\usepackage {xy}
\[\begin{xy}
 0*{DL};<1cm,1cm>*{UR}**@{-}
 ,<0cm,1cm>*{UL};<1cm,0cm>*{DR}**@{=}
\end{xy}\]
\end{document}