proposition: Lemma leB2 x y z t : x <= z -> t <= y -> x `\` y <= z `\` t.
proof: Proof. by move=> /(@leBl t) ? /(@leBr x) /le_trans ->. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice seq) mathcomp(path fintype tuple bigop finset div prime finfun) mathcomp(finset)