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