{--------------------------------------------------------------------
  [filterGt b t] filter all values >[b] from tree [t]
  [filterLt b t] filter all values <[b] from tree [t]
--------------------------------------------------------------------}
filterGt :: Ord a => MaybeS a -> Set a -> Set a
filterGt NothingS t = t
filterGt :: Ord a => MaybeS a -> Set a -> Set a
filterGt NothingS t = t
filterGt NothingS t = t