proposition: Lemma summableD D f g : summable D f -> summable D g -> summable D (f \+ g).
proof: Proof. move=> Df Dg; apply: le_lt_trans (lte_add_pinfty Df Dg). by rewrite -esumD//; apply le_esum => t Dt; exact: lee_abs_add. Qed.
imports: mathcomp(all_ssreflect ssralg ssrnum finmap) mathcomp(mathcomp_extra boolp classical_sets functions) mathcomp(cardinality fsbigop) reals ereal signed topology sequences normedtype numfun