proposition: Lemma discrete_nbhs (p : X) (A : set X) : principal_filter p A -> principal_filter p (principal_filter^~ A).
proof: Proof. by move=> ?; exact/principal_filterP. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed