-- | /O(n+m)/. Is this a subset?
-- @(s1 `isSubsetOf` s2)@ tells whether @s1@ is a subset of @s2@.
isSubsetOf :: Ord a => Set a -> Set a -> Bool
isSubsetOf t1 t2
  = (size t1 <= size t2) && (isSubsetOfX t1 t2)
isSubsetOf :: Ord a => Set a -> Set a -> Bool
isSubsetOf t1 t2
  = (size t1 <= size t2) && (isSubsetOfX t1 t2)
isSubsetOf t1 t2
  = (size t1 <= size t2) && (isSubsetOfX t1 t2)