Definition num_iterations := 1000000000000%positive.(** The simple definition of bounded iteration is:
<<
Fixpoint iterate (niter: nat) (a: A) {struct niter} : option B :=
  match niter with
  | O => None
  | S niter' =>
      match step a with
      | inl b => b
      | inr a' => iterate niter' a'
      end
  end.
>>
  This function is structural recursive over the parameter [niter]
  (number of iterations), represented here as a Peano integer (type [nat]).
  However, we want to use very large values of [niter].  As Peano integers,
  these values would be much too large to fit in memory.  Therefore,
  we must express iteration counts as a binary integer (type [positive]).
  However, Peano induction over type [positive] is not structural recursion,
  so we cannot define [iterate] as a Coq fixpoint and must use
  Noetherian recursion instead. *)

Definition iter_step (x: positive)
                     (next: forall y, Plt y x -> A -> option B)
                     (s: A) : option B :=
  match peq x xH with
  | left EQ => None
  | right NOTEQ =>
      match step s with
      | inl res => Some res
      | inr s'  => next (Pos.pred x) (Ppred_Plt x NOTEQ) s'
      end
  end.Definition iter: positive -> A -> option B := Fix Plt_wf iter_step.(** The [iterate] function is defined as [iter] up to
    [num_iterations] through the loop. *)

Definition iterate := iter num_iterations.

Lemma iter_prop: forall n a b, P a -> iter n a = Some b -> Q b.