signature STREAM =
  sig
    type 'a stream
    val make : ('a * ('a -> 'a)) -> 'a stream
    val next : 'a stream -> ('a * 'a stream)
    val make2 : ('b * ('b -> 'a*'b)) -> 'a stream
  end;
(* One CORRECT Stream implementation *)
structure Stream :> STREAM =
  struct
    datatype 'a stream = Cons of ('a * 'a stream) Thunk.thunk

    fun make (init : 'a, f : 'a -> 'a) : 'a stream =
      Cons(Thunk.make(fn () => (init, make (f init, f))))

    fun make2(init: 'b, f: 'b -> 'a*'b): 'a stream =
      Cons(Thunk.make(fn() =>
                      let val (next_elem, next_state) = f(init) in
                        (next_elem, make2(next_state, f))
                      end))
    fun next (Cons(th) : 'a stream) : 'a * 'a stream =
      Thunk.apply th
  end