--faces
(BackFace _ _ _ _) +++ (BackFace _ _ _ _) = CornerPointsError "illegal BackFace +++ BackFace operation"
(BackFace _ _ _ _) +++ (BackFace _ _ _ _) = CornerPointsError "illegal BackFace +++ BackFace operation"
(BackFace _ _ _ _) +++ (BackFace _ _ _ _) = CornerPointsError "illegal BackFace +++ BackFace operation"