[pstricks] psmatrix and whitespace

[Herbert Voss]

Hi,
could someone explain the behaviour of the following example?

Herbert

\documentclass[a4paper]{article}
\usepackage{pst-node}
\parindent=0pt
\def\r{\rule{1cm}{1pt}}
\begin{document}

The default psmatrix, where the \r macro shows only the
baseline:

\r
\psmatrix
A\\B
\endpsmatrix
\r

Same inside an equation with parenthesis
\[
\r\left(\
\psmatrix
A\\B
\endpsmatrix
\ \right)\r
\]

so far so good, but why does the following construct kills the
vertical whitespace under the B???

\[
\r\left(\
\vcenter{\hbox{$
\psmatrix
A\\B
\endpsmatrix
$}}
\ \right)\r
\]

\end{document}