module type Extensible = 
  sig
    type t
    type 'a constructor
    val create : unit -> 'a constructor
    val inject : 'a constructor -> 'a -> t
    val matches : t -> 'a constructor -> ('a -> 'b) -> (t -> 'b) -> 'b
(*
    val outo : 'a constructor -> t -> 'a option
    val oute : 'a constructor -> t -> 'a
    val is : 'a constructor -> t -> bool
*)
  end;;

module Make (X : sig end) : Extensible = 
 struct
   type t = Obj.t
   type 'a constructor = unit ref
   let create () = ref ()
   let inject c v = Obj.repr (c,v)
   let matches x c f g = 
     let xc, xv = Obj.magic x in if xc == c then f xv else g x
(*
   let outo c x =  out c x (fun z -> Some z) (fun z -> None)
   let oute c x =
     out c x (fun z -> z) (fun x -> raise (Invalid_argument "oute"))
   let is c x = out c x (fun _ -> true) (fun _ -> false)
*)
 end
;;