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\psset{viewpoint=50 40 30 rtp2xyz,Decran=50}
\psset{lightsrc=viewpoint,linewidth=0.5\pslinewidth}
\psSolid[object=grille,base=-4 4 -4 4,action=draw]%
\psSurface[
   fillcolor=cyan!50,
%   tracelignedeniveau=true,
%   hauteurlignedeniveau=5,
%   linewidthlignedeniveau=3,
%   couleurlignedeniveau=blue,
     intersectionplan={[0 0 1 -5]},
     intersectioncolor=(bleu),
     intersectionlinewidth=3,
     intersectiontype=0,
     ngrid=.25 .25,incolor=yellow,
   axesboxed,Zmin=0,Zmax=8,QZ=4](-4,-4)(4,4){%
   y dup mul x dup mul add 4 div }
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