    sig
      type key
      and 'a t
      val create : int -> 'a Hashtbl.S.t
      val clear : 'a Hashtbl.S.t -> unit
      val copy : 'a Hashtbl.S.t -> 'a Hashtbl.S.t
      val add : 'a Hashtbl.S.t -> Hashtbl.S.key -> 'a -> unit
      val remove : 'a Hashtbl.S.t -> Hashtbl.S.key -> unit
      val find : 'a Hashtbl.S.t -> Hashtbl.S.key -> 'a
      val find_all : 'a Hashtbl.S.t -> Hashtbl.S.key -> 'a list
      val replace : 'a Hashtbl.S.t -> Hashtbl.S.key -> 'a -> unit
      val mem : 'a Hashtbl.S.t -> Hashtbl.S.key -> bool
      val iter : (Hashtbl.S.key -> 'a -> unit) -> 'a Hashtbl.S.t -> unit
      val fold : (Hashtbl.S.key -> 'a -> 'b -> 'b) -> 'a Hashtbl.S.t -> 'b -> 'b
    end