fact: Definition nbhs_ {T T'} (ent : set_system (T * T')) (x : T) := filter_from ent (fun A => to_set A x).
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed