// we make a copy here, instead of creating a view of it.
    //
    // TODO: revisit if it's unbearably slow or when GWT supports
    // TreeMap.navigatbleKeySet().
    return ImmutableSortedSet.copyOf(comparator, sortedDelegate.keySet());
  }

  public Comparator<? super K> comparator() {
    return comparator;
  }

  @CheckForNull