proposition: Lemma disj_diffl x y : x `&` y == \bot -> x `\` y = x.
proof: Proof. by move=> dxy; apply/eqP; rewrite eq_diff dxy lexx leUr. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice seq) mathcomp(path fintype tuple bigop finset div prime finfun) mathcomp(finset)