Goal:
  
  A : Type
  B : Type
  f : A → B
  IHn : nat
  l : ∀ l : list A,
        nth_error (map f l) (S IHn) = option_map f (nth_error l (S IHn))
  ============================
   (match map f [] with
    | _ :: _ :: l0 => nth_error l0 IHn
    | _ => None
    end = option_map f None)


DEBUG:Going to execute:
trivial; hnf; intros