proposition: Lemma nnsfun_cover : \big[setU/set0]_(i \in range f) (f @^-1` [set i]) = setT.
proof: Proof. by rewrite fsbig_setU//= -subTset => x _; exists (f x). Qed.
imports: HB(structures) mathcomp(all_ssreflect ssralg ssrnum ssrint interval finmap) mathcomp(mathcomp_extra boolp classical_sets functions) mathcomp(cardinality fsbigop ) signed reals ereal topology normedtype sequences real_interval esum measure lebesgue_measure numfun