%% syntaxe : x y z baseplannormal -> x1 y1 z1 x2 y2 z2
/baseplannormal {
5 dict begin
   /K defpoint3d
   1 0 0 K vectprod3d normalize3d /U defpoint3d
   U norme3d 0 eq {
      0 1 0 K vectprod3d normalize3d /U defpoint3d
   } if
   K U vectprod3d normalize3d /V defpoint3d
   U V
end
} def