private static String badElementIndex(int index, int size, String desc) {
    if (index < 0) {
      return lenientFormat("%s (%s) must not be negative", desc, index);
    } else if (size < 0) {
      throw new IllegalArgumentException("negative size: " + size);
    } else { // index >= size
      return lenientFormat("%s (%s) must be less than size (%s)", desc, index, size);
    }
  }

  /**