proposition: Lemma prod_ball_sym : forall e a b, prod_ball e a b -> prod_ball e b a.
proof: Proof. intros e a b [H1 H2]. split; auto with *. Qed.
imports: CoRN.model.totalorder.QposMinMax CoRN.metric2.Metric CoRN.metric2.Classification CoRN.metric2.UniformContinuity CoRN.metric2.Prelength CoRN.metric2.Complete CoRN.metric2.LocatedSubset