%% 
%%  The LaTeX Graphics Companion, 2ed (first printing May 2007)
%% 
%%  Example 7-4-12 on page 485.
%% 
%%  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{183.83385pt}
\StartShownPreambleCommands
\usepackage[matrix,curve,arrow,tips,frame]{xy}
\StopShownPreambleCommands
\begin{document}
\[\UseTips
\entrymodifiers={++[o][F]}
\xymatrix @-1mm {
 *+\txt{in} \ar[r]
& 1 \ar@(dr,dl)[]^b \ar[r]_a
& 2 \ar@(d,dl)[]^a  \ar[r]_b
& 3 \ar `u[l] `^d[l]_a [l] \ar[r]_b
& *++[o][F=]{4}
  \ar `dl_l[ll]+/d6mm/`l_ul[ll]^a [ll]
  \ar `u^l[lll]+/u1cm/`l^d[lll]_b [lll]
}\]
\end{document}