(BottomRightLine b4 f4) +++ (BottomLeftLine b1 f1) =
    BottomFace b1 f1 b4 f4
(BottomRightLine b4 f4) +++ (BottomLeftLine b1 f1) =
    BottomFace b1 f1 b4 f4
(BottomRightLine b4 f4) +++ (BottomLeftLine b1 f1) =
    BottomFace b1 f1 b4 f4