%% addition de deux vecteurs 3d
/addv3d { %% x1 y1 z1 x2 y2 z2
6 dict begin
   /zp exch def
   /yp exch def
   /xp exch def
   /z exch def
   /y exch def
   /x exch def
   x xp add
   y yp add
   z zp add
end
} def