elim nth; clear nth.
intros.
simpl in |- *; left; trivial.
intros.
simpl in |- *; right; assumption.
Qed.

Lemma nth_app0 :
forall (n : nat) (l0 l1 : list B) (a : B),
my_nth n l0 a -> my_nth n (l0 ++ l1) a.