[pstricks] implicit plots

[Oscar Ecenarro]

Herbert,
I'm trying to draw the equipotential lines of a system of two equal charges at short distances (not a dipolar field), using the implicit plotting scheme that you sugests.
Briefly, the function to be plotted is:
V=k(1/r+1/r')
Placing the origin at the first charge, I use the next definition at the pstVerb environment in Cartesian coordinates:

\pstVerb{%
%------------------------ the implicit function -----------------------
   /func { % parameter is the y value
     /yVal exch def
     1
     x 2 exp
     yVal 2 exp add sqrt div
     1
     x 10 sub 2 exp
    yVal 2 exp add sqrt div
    add
   1 sub
   } def
%------------------------ the implicit function -----------------------
}

The compilation is OK, whith no errors, no warnings, but  when I try to open the ps file, I have a fatal error. This is what apear at the screen:

--- Begin offending input ---
t begin STP newpath 0.8 SLW 0  setgray  90 rotate /n -2 def
/dx 28.45274  def n 0 lt { /dx dx neg def /n n neg def } if /y2 3.0
CLW 2 div add def /y1 y2 neg def /x dx def n { x y1 moveto x y2 lineto
stroke /x x dx add def } repeat end
 
@endspecial 779 1185 a Fa(\000)p Fb(1)779
1421 y Fa(\000)p Fb(2)975 921 y @beginspecial @setspecial
 tx at Dict begin STP newpath 0.8 SLW 1 0 0  setrgbcolor  /ArrowA { moveto
} def /ArrowB { } def /x 0.2 def /x1 8 def /dx x1 x sub 999 div def
/xy { x 28.45274 mul .1  /eps1 0.0001 def /eps2 0.001 def /MaxIter
10 def /y0 exch def /Iter 0 def { y0 func /F exch def F abs eps2 lt
{ exit } if y0 F sub /Phi exch def Phi func /F2 exch def F2 abs eps2
le { exit }{ Phi y0 sub dup mul Phi F2 sub 2 Phi mul sub y0 add div
/Diff exch def y0 Diff sub /y0 exch def Diff abs eps1 le { exit } if
} ifelse /Iter Iter 1 add def Iter MaxIter gt { exit } if } loop y0
  28.45274 mul } def xy moveto  999 1 sub { /x x dx add def xy L  }
repeat /x x1 def xy L  gsave 0.8 SLW 1 0 0  setrgbcolor 0 setlinecap
s
--- End offending input ---
file offset = 39411
gsapi_run_string_continue returns -101

What is the problem? It works fine with others implicit functions, say xy=1.

Oscar Ecenarro
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