\documentclass{ppex}
\nonstopmode
\usepackage [all,poly,knot,dvips]{xy}
\pagestyle{empty}
\setlength\textwidth{183.0pt}
\begin{document}
\ResetPreambleCommands
\ReadyForTheFray
\usepackage [all,poly,knot,dvips]{xy}
\[\UseTips
\renewcommand{\objectstyle}{\scriptstyle}
\renewcommand{\labelstyle}{\scriptstyle}
\begin{xy} /r9mm/:
  \vover\vcross\vcross\vover-
\end{xy}
\hspace{1cm}
\begin{xy} /r9mm/:
  \vunder<><{x_i}|>|{y_i}>>>{z_i}%
  \vtwist|><<\vtwist
  \vunder-<><{x_o}|>|{y_o}>>>{z_o}%
\end{xy}\]
\end{document}
