insert :: (Ord k, Ord v) => k -> v -> Multimap k v -> Multimap k v
insert k v (Multimap m) = Multimap $ Map.alter (inserter v) k m
insert :: (Ord k, Ord v) => k -> v -> Multimap k v -> Multimap k v
insert k v (Multimap m) = Multimap $ Map.alter (inserter v) k m
insert k v (Multimap m) = Multimap $ Map.alter (inserter v) k m