proposition: Lemma subitvPl b1l b2l br : b2l <= b1l -> {subset Interval b1l br <= Interval b2l br}.
proof: Proof. by move=> ?; apply: subitvP; rewrite subitvE lexx andbT. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(div fintype bigop order ssralg finset fingroup) mathcomp(ssrnum)