proposition: Lemma ball_symE (x y : M) (e : R) : ball x e y = ball y e x.
proof: Proof. by rewrite propeqE; split; exact/ball_sym. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed