proposition: Lemma prod_ball_triangle : forall e1 e2 a b c, prod_ball e1 a b -> prod_ball e2 b c -> prod_ball (e1 + e2) a c.
proof: Proof. intros e1 e2 a b c [H1 H2] [H3 H4]. split; eauto with metric. Qed.
imports: CoRN.model.totalorder.QposMinMax CoRN.metric2.Metric CoRN.metric2.Classification CoRN.metric2.UniformContinuity CoRN.metric2.Prelength CoRN.metric2.Complete CoRN.metric2.LocatedSubset