(** ** Properties of [fold_left] *)

Lemma fold_left_right_assoc_eq {A B} {f: A -> B -> A}
(assoc: forall a b c, f (f a b) c = f (f a c) b)
{n} (v: t B n): forall a, fold_left f a v = fold_right (fun x y => f y x) v a.