Fixpoint iter (l : list I) (f : I -> T) :=
  match l with
  | nil => x0
  | cons h l' => op (f h) (iter l' f)
  end.Definition iter' (l : list I) (f : I -> T) :=
  fold_right (fun i acc => op (f i) acc) x0 l.

Lemma iter_cat l1 l2 f : iter (l1 ++ l2) f = op (iter l1 f) (iter l2 f).