(TopRightLine b3 f3) +++ (TopLeftLine b2 f2) =
    TopFace b2 f2 b3 f3
(TopRightLine b3 f3) +++ (TopLeftLine b2 f2) =
    TopFace b2 f2 b3 f3
(TopRightLine b3 f3) +++ (TopLeftLine b2 f2) =
    TopFace b2 f2 b3 f3