Goal:
    A : Type  d : A
  ============================
   (∀ (i : nat) (v : A),
      nth i [] d = v
      → (nth_error [] i = Some v) + (nth_error [] i = None ∧ v = d))


DEBUG:Going to execute:
done
DEBUG LTAC Evaluated term: l
DEBUG LTAC Evaluated term: l