Notation NoDup := (NoDupA E.eq).Notation InA := (InA E.eq).

Lemma fold_rel : forall (A B:Type)(R : A -> B -> Type) (f : elt -> A -> A)(g : elt -> B -> B)(i : A)(j : B)(s : t), R i j -> (forall x a b, In x s -> R a b -> R (f x a) (g x b)) -> R (fold f s i) (fold g s j).