Section Fold_properties.
(** The following lemma has already been proved on Weak Maps, but with one additional hypothesis (some [transpose] fact). *)

Lemma fold_Equal : forall m1 m2 (A:Type)(eqA:A->A->Prop)(st:Equivalence  eqA)
(f:key->elt->A->A)(i:A),
Proper (E.eq==>eq==>eqA==>eqA) f ->
Equal m1 m2 ->
eqA (fold f m1 i) (fold f m2 i).