-6 6 setxrange
-6 6 setyrange

50 setxunit
/ScreenDist .3 def
2 setlinejoin

10 18 10 SetCamPos
0 0 0 SetCamView

/eq1 [0 0 1 -1]  def
/eq2 [0 0 1 0]  def
/eq3 [0 0 1 1]  def

%% plan d equation z = 1
eq1 eq2plan
newplan
dup (blanc) solidputcolors
/Plan1 exch def

%% plan d equation z = 0
eq2 eq2plan
newplan
dup (blanc) solidputcolors
/Plan2 exch def

%% plan d equation z = -1
eq3 eq2plan
newplan
dup (blanc) solidputcolors
/Plan3 exch def

Plan3 drawsolid*
Plan2 drawsolid*
Plan1 drawsolid*
pointilles
Plan3 drawsolid
Plan2 drawsolid