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