Section Fold.
(** Alternative specification via [fold_right] *)
Notation NoDup := (NoDupA E.eq).
Notation InA := (InA E.eq).
(** ** Induction principles for fold (contributed by S. Lescuyer) *)
(** In the following lemma, the step hypothesis is deliberately restricted to the precise set s we are considering. *)
(** Same, with [empty] and [add] instead of [Empty] and [Add]. In this case, [P] must be compatible with equality of sets *)
(** [fold_rec_weak] is a weaker principle than [fold_rec_bis] : the step hypothesis must here be applicable to any [x]. At the same time, it looks more like an induction principle, and hence can be easier to use. *)
(** From the induction principle on [fold], we can deduce some general induction principles on sets. *)

Lemma set_induction :
forall P : t -> Type,
(forall s, Empty s -> P s) ->
(forall s s', P s -> forall x, ~In x s -> Add x s s' -> P s') ->
forall s, P s.