proposition: Lemma clopen_Complement {X:TopologicalSpace} (A : Ensemble X) : clopen A -> clopen (Complement A).
proof: Proof. intros []. split; try assumption. unfold closed. rewrite Complement_Complement. assumption. Qed.
imports: TopologicalSpaces Homeomorphisms SubspaceTopology ZornsLemma(EnsemblesTactics Powerset_facts)