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