proposition: Lemma mx_ball_sym x y (e : R) : mx_ball x e y -> mx_ball y e x.
proof: Proof. by move=> xe_y ??; apply/ball_sym/xe_y. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed