%% 
%%  A DANTE-Edition example
%% 
%%  Example 17-01-20 on page 275.
%% 
%%  Copyright (C) 2011 Herbert Voss
%% 
%%  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.
%% 
%% 
%% ==== 
% Show page(s) 1
%% 
\documentclass[]{article}
\pagestyle{empty}
\setlength\textwidth{153.70511pt}
\setlength\parindent{0pt}

\usepackage{pst-tree,amsmath}

\begin{document}
\pstree[nodesepB=3pt,arrows=->,levelsep=2cm]{%
  \Tdia{$\substack{o|o\\\_}$}}{%
    \TR[edge={\ncbar[angle=180,armB=0.3cm]}]{$x$}
    \TR{$y$}
    \TR[edge={\ncbar[armB=0.3cm]}]{$z$}}
\end{document}