0 0 z a
0 0 z+1/2 a
1/3 2/3 z b
2/3 1/3 z+1/2 b
x y z c
-y x-y z c
-x+y -x z c
y x z+1/2 c
x-y -y z+1/2 c
-x -x+y z+1/2 c