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.

Lemma nth_in : forall (n : nat) (l : list B) (a : B), my_nth n l a -> In a l.