proposition: Lemma image_indic_sub D A : \1_D @` A `<=` ([set 0; 1] : set R).
proof: Proof. by rewrite image_indic; do ![case: ifP=> //= _] => // t []//= ->; [left|right]. Qed.
imports: HB(structures) mathcomp(all_ssreflect ssralg ssrnum ssrint interval finmap) mathcomp(mathcomp_extra boolp classical_sets fsbigop) mathcomp(functions cardinality set_interval) signed reals ereal topology normedtype sequences