proposition: Lemma subset_cons2 s1 s2 x : s1 \subset s2 -> x :: s1 \subset x :: s2.
proof: Proof. by move=> ?; apply/subsetP => y /[!inE]; case: eqP => // _; apply: subsetP. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(path div)