foldr :: (Set a -> b -> b) -> b -> Multimap k a -> b
foldr f b (Multimap m) = Map.foldr f b m
foldr :: (Set a -> b -> b) -> b -> Multimap k a -> b
foldr f b (Multimap m) = Map.foldr f b m
foldr f b (Multimap m) = Map.foldr f b m