* returned first.
   *
   * @since 11.0
   */
  public static <T extends @Nullable Object> UnmodifiableIterator<T> mergeSorted(
      Iterable<? extends Iterator<? extends T>> iterators, Comparator<? super T> comparator) {
    checkNotNull(iterators, "iterators");
    checkNotNull(comparator, "comparator");