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