hedgeDiff :: Ord a => MaybeS a -> MaybeS a -> Set a -> Set a -> Set a
hedgeDiff _   _   Tip           _ = Tip
hedgeDiff :: Ord a => MaybeS a -> MaybeS a -> Set a -> Set a -> Set a
hedgeDiff _   _   Tip           _ = Tip
hedgeDiff _   _   Tip           _ = Tip