[pstricks] node name problem

[Jean-Come Charpentier]

Hello,

To answer at a question on ctt, I have build a code wich have some 
problems. In fact, my problem was a node definition inside a double 
\multido. The code bellow show what I want to make (this code work):

\makeatletter
\let\@nil\relax
\newcount\lastletter
\def\problem#1{%
  \lastletter=1
  \problem@#1\@nil
  \begingroup
  \psset{xunit=8mm,yunit=7mm}
  \begin{pspicture}(0,0)(\lastletter,\lastletter)
    \multido{\n=1+1}{\lastletter}{%
      \multido{\N=1+1}{\lastletter}{%
        \rput(\n.5,\N.5){\psframe(-0.5,0)(0.5,1)%
          \rput(0,0.5){\Rnode{\n,\N}{\@nameuse{l\N}}}%
        }
      }
    }
    \ncarc[arcangle=-60]{->}{1,1}{\the\lastletter,\the\lastletter}
  \end{pspicture}
}
\def\problem@#1#2\@nil{%
  \def\arg{#2}%
  \@namedef{l\the\lastletter}{#1}%
  \ifx\empty\arg
    \let\next\relax
  \else
    \advance\lastletter by1
    \let\next\problem@
  \fi
  \expandafter\next\arg\@nil
}
\problem{ABCDEFGHIJKLMNOP}

In fact, I think that my problem was this: My labels had the syntax 
p(<num>,<num>), in the exemple before, the line
          \rput(0,0.5){\Rnode{\n,\N}{\@nameuse{l\N}}}%
was
          \rput(0,0.5){\Rnode{p(\n,\N)}{\@nameuse{l\N}}}%
and the line
    \ncarc[arcangle=-60]{->}{1,1}{\the\lastletter,\the\lastletter}
was
    \ncarc[arcangle=-60]{->}{p(1,1)}{p(\the\lastletter,\the\lastletter)}

With the syntax "p(<num>,<num>)" TeX was not happy ... very angry in 
fact :-).  Why?  I believed that labels name was *very* free. An other 
behavior is funny: if the label is "p<num>/<num>", TeX is happy, dvips 
is happy but gv is not happy.  Why? Character  "/" is understound as an 
operator by gv? Why parenthesis and comma are prohibit? Is there other 
characters prohibit?

  Jean-Côme Charpentier