proposition: Lemma discrete_sing (p : X) (A : set X) : principal_filter p A -> A p.
proof: Proof. by move=> /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