insertR :: Ord k => k -> v -> MultiMap k v -> MultiMap k v
insertR k v (MM m) = MM (M.alter (Just . maybe (S.singleton v) (S.|> v)) k m)
insertR :: Ord k => k -> v -> MultiMap k v -> MultiMap k v
insertR k v (MM m) = MM (M.alter (Just . maybe (S.singleton v) (S.|> v)) k m)
insertR k v (MM m) = MM (M.alter (Just . maybe (S.singleton v) (S.|> v)) k m)