\documentclass{ppex}
\nonstopmode
\usepackage {xy}
\pagestyle{empty}
\setlength\textwidth{183.0pt}
\begin{document}
\ResetPreambleCommands
\ReadyForTheFray
\usepackage {xy}
\[\begin{xy} 0;<5mm,0mm>:
 0*{DL} ,(2,0)*{DR} ,<0cm,1cm>*{UL} ,(2,2)*{UR}
 ,0+(4,0)="NO"*{\mathtt{DL}}
 ,"NO"+(2,2)="SUR"*{\mathtt{UR}}
 ,"SUR"-<1cm,0cm>*{\mathtt{UL}}
 ,"SUR"-(0,2)*{\mathtt{DR}}
\end{xy}\]
\end{document}