proposition: Lemma ball_itv (x r : R) : (ball x r = `]x - r, x + r[%classic)%R.
proof: Proof. by apply/seteqP; split => y; rewrite /ball/= in_itv/= ltr_distlC. 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