(BackTopLine b2 b3) +++ (FrontTopLine f2 f3) =
    TopFace b2 f2 b3 f3
(BackTopLine b2 b3) +++ (FrontTopLine f2 f3) =
    TopFace b2 f2 b3 f3
(BackTopLine b2 b3) +++ (FrontTopLine f2 f3) =
    TopFace b2 f2 b3 f3