proposition: Lemma closedU (T : topologicalType) (D E : set T) : closed D -> closed E -> closed (D `|` E).
proof: Proof. by rewrite -?openC setCU; exact: openI. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed