hedgeUnion :: Ord a => MaybeS a -> MaybeS a -> Set a -> Set a -> Set a
hedgeUnion _   _   t1  Tip = t1
hedgeUnion :: Ord a => MaybeS a -> MaybeS a -> Set a -> Set a -> Set a
hedgeUnion _   _   t1  Tip = t1
hedgeUnion _   _   t1  Tip = t1