[pstricks] Does psmatrix work with plain TeX?

[David Kanter]

I want to make a flowchart with pstricks and plain TeX. I tried the
minimal example in the User Guide (p. 75) but get compilation errors.
If I LaTeX-ify the document, I get no errors. Interestingly, if I make
a single-column plain TeX psmatrix (no &s), I get no errors:

\input pstricks
\input pst-node

$
\psmatrix[colsep=1cm,rowsep=1cm]
  & A \\
B & E & C \\
  & D &
\endpsmatrix
$
\bye

The errors are:

ERROR: Undefined control sequence.

--- TeX said ---
\pshskip #1->\vrule \@width
                            \z@ \nobreak \hskip #1\hskip \z at skip
\psm at endnode ...pscol >1\relax \pshskip \pscolsep
                                                  \fi \psk at mnodesize \hfil \...
<template> \psm at endnode
                        \endgroup \endtemplate
\psm@@@cr #1->\ifnum 0=`{\fi }{}\cr
                                    \noalign {\global \advance \psrow 1 \glo...
<to be read again>

l.6   & A \\