{
  fun isEven(n) {
    if (n == 0) {
      return true;
    }
    return isOdd(n - 1); # expect runtime error: Undefined variable 'isOdd'.
  }

  fun isOdd(n) {
    if (n == 0) {
      return false;
    }
    return isEven(n - 1);
  }

  isEven(4);
}