proposition: Lemma is_ball0 : is_ball (@set0 R).
proof: Proof. rewrite /is_ball; apply/eqP/seteqP; split => // x; rewrite radius0/=. by rewrite (ball0 _ _).2. Qed.
imports: HB(structures) mathcomp(all_ssreflect ssralg ssrint ssrnum finmap matrix) mathcomp(rat interval zmodp vector fieldext falgebra) mathcomp(mathcomp_extra boolp classical_sets functions) mathcomp(cardinality set_interval Rstruct) ereal reals signed topology prodnormedzmodule