proposition: Lemma lexi_lehead x s1 y s2 : x :: s1 <= y :: s2 :> seq T -> x <= y.
proof: Proof. by rewrite lexi_cons => /andP[]. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice seq) mathcomp(path fintype tuple bigop finset div prime finfun) mathcomp(finset)