{--------------------------------------------------------------------
  Union.
--------------------------------------------------------------------}
-- | The union of a list of sets: (@'unions' == 'foldl' 'union' 'empty'@).
unions :: Ord a => [Set a] -> Set a
unions = foldlStrict union empty
unions :: Ord a => [Set a] -> Set a
unions = foldlStrict union empty
unions = foldlStrict union empty