%% 
%%  Ein Beispiel der DANTE-Edition
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%%  7. Auflage
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%%  Beispiel 16-01-17 auf Seite 303.
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%%  Copyright (C) 2016 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
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\documentclass[]{exaarticle}
\pagestyle{empty}
\setlength\textwidth{145.16928pt}
\setlength\parindent{0pt}
%StartShownPreambleCommands
\usepackage{pst-tree,amsmath}
%StopShownPreambleCommands
\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}