    sig
      type elt
      and t
      val empty : Set.S.t
      val is_empty : Set.S.t -> bool
      val mem : Set.S.elt -> Set.S.t -> bool
      val add : Set.S.elt -> Set.S.t -> Set.S.t
      val singleton : Set.S.elt -> Set.S.t
      val remove : Set.S.elt -> Set.S.t -> Set.S.t
      val union : Set.S.t -> Set.S.t -> Set.S.t
      val inter : Set.S.t -> Set.S.t -> Set.S.t
      val diff : Set.S.t -> Set.S.t -> Set.S.t
      val compare : Set.S.t -> Set.S.t -> int
      val equal : Set.S.t -> Set.S.t -> bool
      val subset : Set.S.t -> Set.S.t -> bool
      val iter : (Set.S.elt -> unit) -> Set.S.t -> unit
      val fold : (Set.S.elt -> 'a -> 'a) -> Set.S.t -> 'a -> 'a
      val for_all : (Set.S.elt -> bool) -> Set.S.t -> bool
      val exists : (Set.S.elt -> bool) -> Set.S.t -> bool
      val filter : (Set.S.elt -> bool) -> Set.S.t -> Set.S.t
      val partition : (Set.S.elt -> bool) -> Set.S.t -> Set.S.t * Set.S.t
      val cardinal : Set.S.t -> int
      val elements : Set.S.t -> Set.S.elt list
      val min_elt : Set.S.t -> Set.S.elt
      val max_elt : Set.S.t -> Set.S.elt
      val choose : Set.S.t -> Set.S.elt
    end