* @return {@code ImmutableListMultimap} mapping the result of evaluating the function {@code
   *     keyFunction} on each value in the input collection to that value
   * @throws NullPointerException if any element of {@code values} is {@code null}, or if {@code
   *     keyFunction} produces {@code null} for any key
   */
  public static <K, V> ImmutableListMultimap<K, V> index(

  }

  public void testConcatNullPointerException() {
    List<Integer> list1 = newArrayList(1);
    List<Integer> list2 = newArrayList(4);

    assertThrows(NullPointerException.class, () -> Iterables.concat(list1, null, list2));
  }

  public void testConcatPeformingFiniteCycle() {
    Iterable<Integer> iterable = asList(1, 2, 3);
    int n = 4;