fact: Definition prod_ball e (a b:X*Y) := ball e (fst a) (fst b) /\ ball e (snd a) (snd b).
imports: CoRN.model.totalorder.QposMinMax CoRN.metric2.Metric CoRN.metric2.Classification CoRN.metric2.UniformContinuity CoRN.metric2.Prelength CoRN.metric2.Complete CoRN.metric2.LocatedSubset