Section ITERATION.
Variables A B: Type.
Variable step: A -> B + A.
Definition B_le (x y: option B) : Prop := x = None \/ y = x.
Definition F_le (x y: A -> option B) : Prop := forall a, B_le (x a) (y a).
Definition F_iter (next: A -> option B) (a: A) : option B :=
match step a with
| inl b => Some b
| inr a' => next a'
end.
Fixpoint iter (n: nat) : A -> option B :=
match n with
| O => (fun a => None)
| S m => F_iter (iter m)
end.

Lemma iter_either:
forall a,
(exists n, exists b, iter n a = Some b) \/
(forall n, iter n a = None).