proposition: Lemma openU (A B : set T) : open A -> open B -> open (A `|` B).
proof: Proof. by rewrite openE => Aop Bop p [/Aop|/Bop]; apply: filterS => ??; [left|right]. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed