(BottomFace _ _ _ _) +++ (BottomFace _ _ _ _) = CornerPointsError "illegal BottomFace +++ BottomFace operation"
(BottomFace _ _ _ _) +++ (BottomFace _ _ _ _) = CornerPointsError "illegal BottomFace +++ BottomFace operation"
(BottomFace _ _ _ _) +++ (BottomFace _ _ _ _) = CornerPointsError "illegal BottomFace +++ BottomFace operation"