(** * [fold] *)

Fixpoint fold (A:Type)(f:key->elt->A->A)(m:t elt) (acc : A) {struct m} :  A :=
  match m with
   | nil => acc
   | (k,e)::m' => fold f m' (f k e acc)
  end.

Lemma fold_1 : forall m (A:Type)(i:A)(f:key->elt->A->A), fold f m i = fold_left (fun a p => f (fst p) (snd p) a) (elements m) i.