proposition: Lemma within_subset B F : Filter F -> A `<=` B -> within A F `=>` within B F.
proof: Proof. move=> FF AsubB W; rewrite /within/=; apply: filter_app; rewrite nbhs_simpl. by apply: filterE => ? + ?; apply; exact: AsubB. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed