proposition: Lemma cluster_nbhs t : cluster (nbhs t) t.
proof: Proof. by move=> A B /nbhs_singleton At /nbhs_singleton Bt; exists t. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed