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

Lemma fold_1 :
forall (s:t elt)(Hs:bst s)(A : Type)(i:A)(f : key -> elt -> A -> A),
fold f s i = fold_left (fun a p => f p#1 p#2 a) (elements s) i.