*
   * <p>Retrieving a single element takes approximately O(log(M)) time, where M is the number of
   * iterators. (Retrieving all elements takes approximately O(N*log(M)) time, where N is the total
   * number of elements.)
   */
  private static class MergingIterator<T extends @Nullable Object> extends UnmodifiableIterator<T> {
    final Queue<PeekingIterator<T>> queue;

    public MergingIterator(