(* IF NO DART IS IN s, s IS Vs: *)

(* Specifications: *)
Lemma not_exds_Vs: forall s:set,
(forall z:dart, ~exds s z) -> s = Vs.
Lemma not_exds_diff: forall(s:set)(z:dart),
~exds s z ->
forall(t:dart), exds s t -> z <> t.