proposition: Lemma ball_norm_sym x y (e : R) : ball_norm x e y -> ball_norm y e x.
proof: Proof. by rewrite /ball_norm/= -opprB normrN. 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