proposition: Lemma mx_ball_triangle x y z (e1 e2 : R) : mx_ball x e1 y -> mx_ball y e2 z -> mx_ball x (e1 + e2) z.
proof: Proof. by move=> xe1_y ye2_z ??; apply: ball_triangle; [apply: xe1_y| apply: ye2_z]. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed