intros.
generalize dependent i.
generalize dependent v.
induction xs;
intros.
rewrite Coqlib.nth_error_nil in H.
discriminate.
destruct i; simpl in *;
now auto.
Qed.

Lemma app_assoc_l:
forall (A : Type) (l m n : seq.seq A),
l ++ m ++ n = l ++ (m ++ n).