* sorting all elements at once.
   *
   * <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> {

    Integer lastItem = 0;
    for (Integer tmp : mmHeap) {
      lastItem = tmp;
    }
    assertEquals((Integer) 30, lastItem);
  }

  /**
   * This tests a special case where removeAt has to trickle an element first down one level from a
   * min to a max level, then up one level above the index of the removed element. It also tests
   * that skipMe in the iterator plays nicely with forgetMeNot.
   */