lookup :: Ord k => k -> Multimap k v -> Set v
lookup k (Multimap m) = Maybe.fromMaybe Set.empty (Map.lookup k m)
lookup :: Ord k => k -> Multimap k v -> Set v
lookup k (Multimap m) = Maybe.fromMaybe Set.empty (Map.lookup k m)
lookup k (Multimap m) = Maybe.fromMaybe Set.empty (Map.lookup k m)