proposition: Lemma discrete_set1 (x : X) : nbhs x [set x].
proof: Proof. by apply: open_nbhs_nbhs; split => //; exact: discrete_open. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed