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

Theorem fold_rec : forall (A:Type)(P : t -> A -> Type)(f : elt -> A -> A)(i:A)(s:t), (forall s', Empty s' -> P s' i) -> (forall x a s' s'', In x s -> ~In x s' -> Add x s' s'' -> P s' a -> P s'' (f x a)) -> P s (fold f s i).