module type S = sig
  type elt 
  type 'a node = Nil | Cons of elt * 'a
  type t
  val hash : t -> int
  val uid : t -> int
  val equal : t -> t -> bool
  val nil : t
  val node : t -> t node
  val cons : elt -> t -> t
  val hd : t -> elt
  val tl : t -> t
  val fold : (elt -> 'a -> 'a) -> t -> 'a -> 'a
  val map : (elt -> elt) -> t -> t
  val iter : (elt -> 'a) -> t -> unit
  val rev : t -> t
  val rev_map : (elt -> elt) -> t -> t
  val length : t -> int
end

module Make (H : Hcons.S) : S with type elt = H.t