proposition: Lemma prod_ball_triangle : forall (x y z : U * V) (e1 e2 : R), prod_ball x e1 y -> prod_ball y e2 z -> prod_ball x (e1 + e2) z.
proof: Proof. intros x y z e1 e2 [H1 H2] [H3 H4]. split ; eapply ball_triangle ; eassumption. Qed.
imports: Coq(Reals Psatz ssreflect) Rcomplements Rbar Markov Iter Lub