proposition: Lemma prod_ball_sym x y (eps : R) : prod_ball x eps y -> prod_ball y eps x.
proof: Proof. by move=> [bxy1 bxy2]; split; apply: 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