removePlusses :: Term -> S.Set Term
removePlusses (Plus p1 p2) = removePlusses p1 `S.union` removePlusses p2
removePlusses :: Term -> S.Set Term
removePlusses (Plus p1 p2) = removePlusses p1 `S.union` removePlusses p2
removePlusses (Plus p1 p2) = removePlusses p1 `S.union` removePlusses p2