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.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.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.

Lemma inv_nthS : forall (n : nat) (a : B) (l : list B) (b : B), my_nth (S n) (a :: l) b -> my_nth n l b.