proposition: Lemma withinT F D : Filter F -> within D F D.
proof: Proof. by move=> FF; rewrite /within/=; apply: filterE. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed