proposition: Lemma summableB D f g : summable D f -> summable D g -> summable D (f \- g).
proof: Proof. by move=> Df; rewrite summableN; exact: summableD. 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