proposition: Lemma ball_sym (x y : M) (e : R) : ball x e y -> ball y e x.
proof: Proof. exact: ball_sym_subproof. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed