@Override
  Iterator<C> createColumnKeyIterator() {
    Comparator<? super C> comparator = columnComparator();

    Iterator<C> merged =
        Iterators.mergeSorted(
            Iterables.transform(
                backingMap.values(), (Map<C, V> input) -> input.keySet().iterator()),
            comparator);

    return new AbstractIterator<C>() {

  @Override
  Iterator<C> createColumnKeyIterator() {
    Comparator<? super C> comparator = columnComparator();

    Iterator<C> merged =
        Iterators.mergeSorted(
            Iterables.transform(
                backingMap.values(), (Map<C, V> input) -> input.keySet().iterator()),
            comparator);

    return new AbstractIterator<C>() {

   * <p>For any equivalent elements across all {@code iterables}, it is undefined which element is
   * returned first.
   *
   * @since 11.0
   */
  public static <T extends @Nullable Object> Iterable<T> mergeSorted(
      final Iterable<? extends Iterable<? extends T>> iterables,
      final Comparator<? super T> comparator) {
    checkNotNull(iterables, "iterables");
    checkNotNull(comparator, "comparator");

   * <p>For any equivalent elements across all {@code iterables}, it is undefined which element is
   * returned first.
   *
   * @since 11.0
   */
  public static <T extends @Nullable Object> Iterable<T> mergeSorted(
      final Iterable<? extends Iterable<? extends T>> iterables,
      final Comparator<? super T> comparator) {
    checkNotNull(iterables, "iterables");
    checkNotNull(comparator, "comparator");

   * 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");

   * 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");