(** * Fold *)
Definition fold' (A : Type) (f : key -> elt -> A -> A)(s : t elt) :=
L.fold f (elements s).

Lemma fold_equiv_aux :
forall (A : Type) (s : t elt) (f : key -> elt -> A -> A) (a : A) acc,
L.fold f (elements_aux acc s) a = L.fold f acc (fold f s a).