(TopFace b2 f2 b3 f3) +++ (TopLeftLine b2t f2t) =
    (TopFace b2t f2t b2 f2)
(TopFace b2 f2 b3 f3) +++ (TopLeftLine b2t f2t) =
    (TopFace b2t f2t b2 f2)
(TopFace b2 f2 b3 f3) +++ (TopLeftLine b2t f2t) =
    (TopFace b2t f2t b2 f2)