proposition: Lemma ball_norm_center (x : R) (e : K) : 0 < e -> ball_ Num.norm x e x.
proof: Proof. by move=> ? /=; rewrite subrr normr0. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap generic_quotient) mathcomp(boolp classical_sets functions) mathcomp(cardinality mathcomp_extra fsbigop) reals signed