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

Lemma fold_0 : forall s (A : Type) (i : A) (f : elt -> A -> A), exists l : list elt, NoDup l /\ (forall x : elt, In x s <-> InA x l) /\ fold f s i = fold_right f i l.