[OS X TeX] pstricks question

[Sergei Mariev]

I am trying to typeset a tree using  pstricks. I have the following code:


   \pstree[nodesep=2pt]{\Tr{EI}}{\Tr{P} \pstree{\Tr{a}}{\Tr{S}}}



And I am getting the following error-message:


PSTricks error.  See User's Guide for further information.
                  Type  H <return>  for immediate help.
! Graphics parameter `nodesep=2pt' not defined..
\@pstrickserr ... immediate help.}\errmessage {#1}
                                                   \endgroup
l.7     \pstree[nodesep=2pt]{\Tr{EI}}
                                      {\Tr{P} \pstree{\Tr{a}}{\Tr{S}}}
? x


In other words, it does not recognize the optional parameter 
[nodesep=2pt] ...??? Do I need to load a  special package for this? 
(Initially I tried loading just 
\usepackage[]{pstricks,pst-plot,pst-node,pst-tree}) I also tried 
\usepackage{pst-all} ... but it still does not seem to recognize this 
optional nodesep parameter! Does anyone know what I am doing wrong?

Thanks for any advice,

Sergei Mariev
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