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