Goal:
    A : Type  B : Type  f : A → B
  ============================
   (∀ n : nat,
      (∀ l : list A,
         nth_error (map f l) (S n) = option_map f (nth_error l (S n)))
      → ∀ l : list A,
          nth_error (map f l) (S (S n)) =
          option_map f (nth_error l (S (S n))))


DEBUG:Going to execute:
trivial; hnf; intros