proposition: Lemma indicE A (x : T) : \1_A x = (x \in A)%:R :> R.
proof: Proof. by []. 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