Section BasicProperties.
(** Some old specifications written with boolean equalities. *)
Variable s s' s'': t.
Variable x y z : elt.
(** properties of [mem] *)
(** Properties of [equal] *)
(* Properties of [subset] *)
(** Properties of [choose] *)
(** Properties of [add] *)
(** Properties of [remove] *)
(** Properties of [is_empty] *)
(** Properties of [singleton] *)
(** Properties of [union] *)

Lemma union_add:
equal (union (add x s) s') (add x (union s s'))=true.