(TopFace b2 f2 b3 f3) +++ (FrontTopLine f2a f3a) =
    TopFace {b2=f2, b3=f3, f2=f2a, f3=f3a}
(TopFace b2 f2 b3 f3) +++ (FrontTopLine f2a f3a) =
    TopFace {b2=f2, b3=f3, f2=f2a, f3=f3a}
(TopFace b2 f2 b3 f3) +++ (FrontTopLine f2a f3a) =
    TopFace {b2=f2, b3=f3, f2=f2a, f3=f3a}