union :: Ord k => MultiMap k v -> MultiMap k v -> MultiMap k v
union (MM m1) (MM m2) = MM (M.unionWith (S.><) m1 m2)
union :: Ord k => MultiMap k v -> MultiMap k v -> MultiMap k v
union (MM m1) (MM m2) = MM (M.unionWith (S.><) m1 m2)
union (MM m1) (MM m2) = MM (M.unionWith (S.><) m1 m2)