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