%% 
%%  The LaTeX Graphics Companion, 2ed (first printing May 2007)
%% 
%%  Example 7-4-2 on page 479.
%% 
%%  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{159.83385pt}
\StartShownPreambleCommands
\usepackage[curve,arrow,frame]{xy}
\StopShownPreambleCommands
\begin{document}
\[\begin{xy} <1cm,0cm>:
             (0,0)*+@{*}="a"*+!DL{\mathrm{A}},"a"
 \ar@(dr,dl)
 \ar@{~>}    (2,0)*+@{*}="b"*+!DR{\mathrm{B}},"b"
 \ar@(r,d)   "b";"b"
 \ar@{<->}   "b";(2,2)*+@{*}="c"*+!UR{\mathrm{C}},"c"
 \ar@(u,r)   "c";"c" |*=<2pt>[o][F]{}
 \ar@(ul,dr) "c"; (0,2)*+@{*}="d"*+!U{\mathrm{D}},"d"
 \ar@(ul,ur) "d";"d"
 \ar@{.>}@`{(-0.8,1.5),(0.,1.0),(-0.8,0.5)} "d";
\end{xy}\]
\end{document}