proposition: Lemma ball_norm_symmetric (x y : R) (e : K) : ball_ Num.norm x e y -> ball_ Num.norm y e x.
proof: Proof. by rewrite /= distrC. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed