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