proposition: Lemma ball_triangle (y x z : M) (e1 e2 : R) : ball x e1 y -> ball y e2 z -> ball x (e1 + e2) z.
proof: Proof. exact: ball_triangle_subproof. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed