proposition: Lemma measurable_ysection A y : measurable A -> measurable (ysection A y).
proof: Proof. move=> mA; rewrite (ysection_indic R) -(setTI (_ @^-1` _)). exact: measurableT_comp. 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