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_inter : forall i s s',
eqA (fold f (union s s') (fold f (inter s s') i))
(fold f s (fold f s' i)).