usereg
[ (-1,-1,0) (1,-1,0)
(1,1,0) (-1,1,0) ] !poly
:poly 1 poly2doubleface
dup edgemate exch
1 1 extrude-simple !f0 !f1
:poly { 0.5 mul } map reverse
5 poly2doubleface
-1 1 extrude-simple
!r0 !r1
:r0 :f0 killFmakeRH
:r1 :f1 killFmakeRH