%% 
%%  The LaTeX Graphics Companion, 2ed (first printing May 2007)
%% 
%%  Example 7-4-39 on page 506.
%% 
%%  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[curve,knot,graph,dvips]{xy}
\StopShownPreambleCommands
\begin{document}
\[\xygraph{!{0;/r10mm/:}
         !{\vover}
     [u] !{\hcap[-2]}
     [d] !{\vover-}
   [ruu] !{\hcap[2]}
}\]
\[\begin{xy} 0;/r10mm/:
  ,\hcap[-2]\vunder\vunder-
  ,+(1,2),\hcap[2]
\end{xy}\]
\end{document}