--lines
(BackBottomLine _ _) +++ (BackBottomLine _ _) = CornerPointsError "illegal BackBottomLine +++ BackBottomLine operation"
(BackBottomLine _ _) +++ (BackBottomLine _ _) = CornerPointsError "illegal BackBottomLine +++ BackBottomLine operation"
(BackBottomLine _ _) +++ (BackBottomLine _ _) = CornerPointsError "illegal BackBottomLine +++ BackBottomLine operation"