TikZ / Flussdiagramm: Symbol für Unterprogramm

\documentclass[border=5mm]{standalone}
\usepackage{selinput}
\SelectInputMappings{adieresis={ä},germandbls={ß}}
\usepackage{tikz}
\usetikzlibrary{matrix,shapes}

\tikzset{
treenode/.style = {
shape=rectangle, 
draw, anchor=center,
%text width=5em, 
align=center,
draw=cyan, very thick
%inner sep=1ex
},
startstop/.style      = {treenode, rounded corners=7pt},
process/.style      = {treenode},
unterprogramm/.style = {treenode,
  path picture={\draw
    ([xshift=1mm]path picture bounding box.north west)--([xshift=1mm]path picture bounding box.south west)
    ([xshift=-1mm]path picture bounding box.north east)--([xshift=-1mm]path picture bounding box.south east)
  ;}
},
decision/.style = {treenode, diamond},
output/.style={trapezium,trapezium left angle=60, trapezium right angle=120, font=\sffamily\slshape\small},
input/.style={trapezium,trapezium left angle=120, trapezium right angle=60},
}

\begin{document}
\begin{tikzpicture}[font=\sffamily\small,
]

\def\Start{|[startstop, text width=2cm]| Start}
\def\Entscheidung{|[decision,  text width=1.75cm]| Entscheidung}
\def\Prozess{|[process,  text width=1.75cm]| Prozess}
\def\Eingabe{|[input, text width=1.5cm]| Eingabe}
\def\Ausgabe{|[output, text width=2cm]| Ausgabe}
\def\Unterprogramm{|[unterprogramm,text width=1.75cm]|{Unter-\\programm}}

\matrix (chart)
[
matrix of nodes,
%nodes in empty cells,
column sep      = 3em,
row sep         = 3ex,
nodes={process}       % default
]
{
\Start & \Entscheidung & \Prozess  \\
\Eingabe & \Ausgabe & \Unterprogramm  \\
};
\end{tikzpicture}
\end{document}