return CollectCollectors.toImmutableSortedMap(
        comparator, keyFunction, valueFunction, mergeFunction);
  }

  /*
   * TODO(kevinb): Confirm that ImmutableSortedMap is faster to construct and
   * uses less memory than TreeMap; then say so in the class Javadoc.
   */
  private static final Comparator<?> NATURAL_ORDER = Ordering.natural();

  private static final ImmutableSortedMap<Comparable<?>, Object> NATURAL_EMPTY_MAP =

    int skipped = advance(iterator, position);
    if (!iterator.hasNext()) {
      throw new IndexOutOfBoundsException(
          "position ("
              + position
              + ") must be less than the number of elements that remained ("
              + skipped
              + ")");
    }
    return iterator.next();
  }

  /**