%% 
%%  The LaTeX Graphics Companion, 2ed (first printing May 2007)
%% 
%%  Example 7-4-36 on page 504.
%% 
%%  Copyright (C) 2007 Michel Goossens, Frank Mittelbach, Denis Roegel, Sebastian Rahtz, Herbert Vo\ss 
%% 
%%  It may be distributed and/or modified under the conditions
%%  of the LaTeX Project Public License, either version 1.3
%%  of this license or (at your option) any later version.
%% 
%%  See http://www.latex-project.org/lppl.txt for details.
%% 

\documentclass{ttctexa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{207.83385pt}
\StartShownPreambleCommands
\usepackage[all,poly,knot,dvips]{xy}
\StopShownPreambleCommands
\begin{document}
\[\UseTips
\renewcommand{\objectstyle}{\scriptstyle}
\renewcommand{\labelstyle}{\scriptstyle}
\begin{xy} /r9mm/:
  \vover\vcross\vcross\vover-
\end{xy}
\hspace{1cm}
\begin{xy} /r9mm/:
  \vunder<><{x_i}|>|{y_i}>>>{z_i}%
  \vtwist|><<\vtwist
  \vunder-<><{x_o}|>|{y_o}>>>{z_o}%
\end{xy}\]
\end{document}