Section My_Nth.
Variable B : Set.
Inductive my_nth : nat -> list B -> B -> Prop :=
| My_NthO : forall (l : list B) (a : B), my_nth 0 (a :: l) a
| My_NthS :
forall (n : nat) (l : list B) (b : B),
my_nth n l b -> forall a : B, my_nth (S n) (a :: l) b.
intros.
inversion H.
Qed.
intros.
inversion H.
trivial.
Qed.
intros.
inversion H.
assumption.
Qed.
intros P base step n.
elim n; clear n.
intros l; case l; clear l.
intros y nth; elimtype False.
apply (inv_nth_nil 0 y nth).
intros a l b nth.
rewrite (inv_nthO a l b nth).
apply base.
intros n ih l.
case l; clear l.
intros y nth; elimtype False.
apply (inv_nth_nil (S n) y nth).
intros a l b nth.
apply step.
apply (inv_nthS n a l b nth).
apply ih.
apply (inv_nthS n a l b nth).
Qed.
intros n l a nth.
elim nth; clear nth.
intros.
simpl in |- *; left; trivial.
intros.
simpl in |- *; right; assumption.
Qed.
intros n l0.
generalize n; clear n.
elim l0; clear l0.
intros n l1 a nth; elimtype False.
apply (inv_nth_nil n a nth).
intros a0 l0 ih n.
case n; clear n.
intros l1 a nth.
rewrite (inv_nthO a0 l0 a nth).
simpl in |- *; apply My_NthO.
intros n l1 a nth.
simpl in |- *; apply My_NthS.
apply ih.
apply (inv_nthS n a0 l0 a nth).
Qed.

Lemma nth_app1 :
forall (n : nat) (l0 l1 : list B) (a : B),
my_nth n l1 a -> my_nth (length l0 + n) (l0 ++ l1) a.