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

Lemma fold_diff_inter : forall i s s',
eqA (fold f (diff s s') (fold f (inter s s') i)) (fold f s i).