(TopLeftLine b2 f2) +++ (BottomLeftLine b1 f1) =
    LeftFace b1 b2 f1 f2
(TopLeftLine b2 f2) +++ (BottomLeftLine b1 f1) =
    LeftFace b1 b2 f1 f2
(TopLeftLine b2 f2) +++ (BottomLeftLine b1 f1) =
    LeftFace b1 b2 f1 f2