(RightFace b3f b4f f3f f4f) +++ (BottomRightLine b4l f4l) =
  (RightFace b4f b4l f4f f4l)
(RightFace b3f b4f f3f f4f) +++ (BottomRightLine b4l f4l) =
  (RightFace b4f b4l f4f f4l)
(RightFace b3f b4f f3f f4f) +++ (BottomRightLine b4l f4l) =
  (RightFace b4f b4l f4f f4l)