[pstricks] Problem with Kreisel example from PSTRICKS site

[Bruce Burlton]

I am drawing several diagrams that involve showing angular velocities, represented by partial circular arcs, much as are shown in the spinning top example in the Kreisel.tex file, implemented by the small macro \psRotation.  However, when I copy the following code

\makeatletter
\def\psRotation#1(#2,#3)#4{%
  \rput{#1}(#2,#3){%
    \psellipticarc[linewidth=.4pt]{->}(0,-0.1)(0.6,0.15){120}{70}
    \ifdim#1pt>\z@\rput[l]{*0}(0.675,0){#4}\else\rput[l](0.675,0){#4}\fi
  }%
}
\makeatother
\pagestyle{empty}

into my file and try to use it, I get un undefined control sequence \z

When I look at the code in the Kreisel example, it looks undefined to me, but the Kreisel example compiles and works.  To make things a bit more complicated, I am using Beamer and PSforPDF.  The above code is included in the first PSforPDF call in the preamble just after the 

\usepackage{pstricks,pst-3dplot,pst-node,pstricks-add}

line


I am confused.  As an aside, this would be a useful macro to have in the PSTRICKS (or other) package

Thanks
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