*     keyFunction} produces {@code null} for any key
   * @since 10.0
   */
  public static <K, V> ImmutableListMultimap<K, V> index(
      Iterator<V> values, Function<? super V, K> keyFunction) {
    checkNotNull(keyFunction);
    ImmutableListMultimap.Builder<K, V> builder = ImmutableListMultimap.builder();
    while (values.hasNext()) {
      V value = values.next();
      checkNotNull(value, values);