pstricks problem?

Hello all,

I guess this is really a pstricks problem, but I thought I'd 
check here anyway, in case anyone has any insight.

According to the pstricks documentation, rput does not occupy
any space. However, the amount of vertical space before the
first item in the following two slides is different (it's 
larger with the reference command). 

Am I doing something wrong? I'd greatly appreciate any help.

Mandar.

ps: my pstricks version is dated '97 
    ([1997/03/25 package wrapper for PSTricks pstricks.tex] to be precise)

---

   \documentclass[pdf,slideColor,nocolorBG]{prosper}
   
   \newcommand{\reference}[1] {\rput{90}(-0.7,-3.2){\parbox{7cm}{\begin{center}\fbox{\rm #1}\end{center}}}}
   
   \begin{document}
   
   \begin{slide}{Introduction}
   \reference{Vahalia 10.1, 10.2}
   \begin{itemize}
   \item Motivation: sharing of files between networked computers
     \begin{itemize}
     \item initially: explicit copying of entire files between machines
   
       e.g. uucp, ftp
     \item currently: transparent access to files on remote machines over
       network
     \end{itemize}
   \end{itemize}
   \end{slide}
   
   \begin{slide}{Introduction}
   %\reference{Vahalia 10.1, 10.2}
   \begin{itemize}
   \item Motivation: sharing of files between networked computers
     \begin{itemize}
     \item initially: explicit copying of entire files between machines
   
       e.g. uucp, ftp
     \item currently: transparent access to files on remote machines over
       network
     \end{itemize}
   \end{itemize}
   \end{slide}

   \end{document}
   


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