proposition: Lemma clopenU {T} (A B : set T) : clopen A -> clopen B -> clopen (A `|` B).
proof: Proof. by case=> ? ? [] ? ?; split; [exact: openU | exact: closedU]. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed