Definition recursion {A} : A -> (N -> A -> A) -> N -> A :=
  peano_rect (fun _ => A).#[global]
Instance recursion_wd {A} (Aeq : relation A) :
 Proper (Aeq==>(Logic.eq==>Aeq==>Aeq)==>Logic.eq==>Aeq) recursion.

Theorem iter_invariant : forall (n:N) (A:Type) (f:A -> A) (Inv:A -> Prop), (forall x:A, Inv x -> Inv (f x)) -> forall x:A, Inv x -> Inv (iter n f x).