%% 
%%  The LaTeX Graphics Companion, 2ed (first printing May 2007)
%% 
%%  Example 6-3-24 on page 377.
%% 
%%  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{pstricks,pst-tree,amsmath}
\StopShownPreambleCommands
\begin{document}
\pstree[nodesepB=3pt,arrows=->,levelsep=2cm]{%
  \Tdia{\begin{tabular}{c}o|o\\\_\end{tabular}}}{%
    \TR[edge={\ncbar[angle=180,armB=0.3cm]}]{x}
    \TR{y}
    \TR[edge={\ncbar[armB=0.3cm]}]{z}
  }
\end{document}