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