Goal:
    A : Type  B : Type  P : nat → A → B → Type  n : nat  x : A  n0 : nat
  ============================
   (None = Some x → ∑ c : B, nth_error [] n = Some c × P (n0 + n)%nat x c)


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