Section Fold_More.
Variables (A:Type)(eqA:A->A->Prop)(st:Equivalence eqA).
Variables (f:elt->A->A)(Comp:Proper (E.eq==>eqA==>eqA) f)(Ass:transpose eqA f).
(** ** Fold is a morphism *)
(** ** Fold and other set operators *)

Lemma fold_union: forall i s s',
(forall x, ~(In x s/\In x s')) ->
eqA (fold f (union s s') i) (fold f s (fold f s' i)).