proposition: Lemma sub_stack e1 e2 : subenv e1 e2 -> stack e1 \subset stack e2.
proof: Proof. move=> sube12; apply/subsetP=> x; rewrite !inE => x_stack. by rewrite (sub_vnum sube12)// (leq_trans x_stack)// leq_sn. Qed.
imports: mathcomp(all_ssreflect) extra_nocolors