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