proposition: Lemma cardinalO_empty : forall X:Ensemble U, cardinal U X 0 -> X = Empty_set U.
proof: Proof. intros X H; apply (cardinal_invert U X 0); trivial with sets. Qed.
imports: Finite_sets Constructive_sets Classical Classical_sets Powerset Powerset_facts Powerset_Classical_facts PeanoNat