(** From the induction principle on [fold], we can deduce some general induction principles on sets. *)

(* Specifications: *)
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.
Lemma set_induction_bis :
forall P : t -> Type,
(forall s s', s [=] s' -> P s -> P s') ->
P empty ->
(forall x s, ~In x s -> P s -> P (add x s)) ->
forall s, P s.
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.
Lemma set_induction_bis :
forall P : t -> Type,
(forall s s', s [=] s' -> P s -> P s') ->
P empty ->
(forall x s, ~In x s -> P s -> P (add x s)) ->
forall s, P s.