module type Dyn = sig
  type 'a brand
  type t
  val make_brand : unit -> 'a brand
  val pack : 'a brand -> 'a -> t 
  val unpack : 'a brand -> t -> 'a option
  val null : t
end

module Dyn : Dyn = struct
  type 'a brand = ('a -> exn) * (exn -> 'a option)
  type t = exn
  let make_brand : unit -> 'a brand = function () ->
    let module X = struct
      exception E of 'a
      let pack x = E x
      let unpack = function
	| E x -> Some x
	| _ -> None
    end in
    X.pack, X.unpack
  let pack = fst
  let unpack = snd
  exception Null
  let null = Null
end

module Dyn : Dyn = struct
  type 'a brand = ('a option) ref
  type t = unit -> unit
  let make_brand () =
    ref None
  let pack b x () =
    b := Some x
  let unpack b f =
    b := None;
    f();
    !b
  let null () =
    ()
end