proposition: Lemma trivIset_preimage1 {aT rT} D (f : aT -> rT) : trivIset D (fun x => f @^-1` [set x]).
proof: Proof. by move=> y z _ _ [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