val inter : ?compare:('a -> 'a -> int) -> 'a list -> 'a list -> 'a listval union : ?compare:('a -> 'a -> int) -> 'a list -> 'a list -> 'a listval subset : ?compare:('a -> 'a -> int) -> 'a list -> 'a list -> boolval equal : ?compare:('a -> 'a -> int) -> 'a list -> 'a list -> boolval is_proper_set : 'a list -> bool