proposition: Lemma itv_boundlr bl br x : (x \in Interval bl br) = (bl <= BLeft x) && (BRight x <= br).
proof: Proof. by []. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(div fintype bigop order ssralg finset fingroup) mathcomp(ssrnum)