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