proposition: Lemma in_itvI x i1 i2 : x \in i1 `&` i2 = (x \in i1) && (x \in i2).
proof: Proof. exact: lexI. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(div fintype bigop order ssralg finset fingroup) mathcomp(ssrnum)