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