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 *)
End Fold_More.

Lemma fold_plus :
forall s p, fold (fun _ => S) s p = fold (fun _ => S) s 0 + p.