proposition: Lemma prod_ball_refl : forall (e:Q) a, 0 <= e -> prod_ball e a a.
proof: Proof. intros e a. split; apply ball_refl; exact H. Qed.
imports: CoRN.model.totalorder.QposMinMax CoRN.metric2.Metric CoRN.metric2.Classification CoRN.metric2.UniformContinuity CoRN.metric2.Prelength CoRN.metric2.Complete CoRN.metric2.LocatedSubset