}.test();
  }

  public void testConcatPartiallyAdvancedSecond() {
    Iterator<String> itr1 = Iterators.concat(singletonIterator("a"), Iterators.forArray("b", "c"));
    assertEquals("a", itr1.next());
    assertEquals("b", itr1.next());
    Iterator<String> itr2 = Iterators.concat(singletonIterator("d"), itr1);
    assertEquals("d", itr2.next());
    assertEquals("c", itr2.next());
  }

  }

  public void testConcatPartiallyAdvancedSecond() {
    Iterator<String> itr1 =
        Iterators.concat(Iterators.singletonIterator("a"), Iterators.forArray("b", "c"));
    assertEquals("a", itr1.next());
    assertEquals("b", itr1.next());
    Iterator<String> itr2 = Iterators.concat(Iterators.singletonIterator("d"), itr1);
    assertEquals("d", itr2.next());
    assertEquals("c", itr2.next());
  }

    private final Deque<Iterator<T>> stack;

    PreOrderIterator(T root) {
      this.stack = new ArrayDeque<>();
      stack.addLast(Iterators.singletonIterator(checkNotNull(root)));
    }

    @Override
    public boolean hasNext() {
      return !stack.isEmpty();
    }

    @Override
    public T next() {

   * <p>The {@link Iterable} equivalent of this method is {@link Collections#singleton}.
   */
  public static <T extends @Nullable Object> UnmodifiableIterator<T> singletonIterator(
      @ParametricNullness T value) {
    if (value != null) {
      return new SingletonIterator<>(value);
    }
    @SuppressWarnings("nullness") // For `value` to be null, T must be a nullable type.

    Multiset<String> multiset = ImmutableMultiset.copyOf(iterator);
    assertTrue(multiset.isEmpty());
  }

  public void testCopyOf_iterator_oneElement() {
    Iterator<String> iterator = Iterators.singletonIterator("a");
    Multiset<String> multiset = ImmutableMultiset.copyOf(iterator);
    assertEquals(HashMultiset.create(asList("a")), multiset);
  }

  public void testCopyOf_iterator_general() {

    List<String> list = ImmutableList.copyOf(iterator);
    assertEquals(Collections.emptyList(), list);
  }

  public void testCopyOf_iterator_oneElement() {
    Iterator<String> iterator = Iterators.singletonIterator("a");
    List<String> list = ImmutableList.copyOf(iterator);
    assertEquals(Collections.singletonList("a"), list);
  }

  public void testCopyOf_iterator_general() {

    Multiset<String> multiset = ImmutableSortedMultiset.copyOf(iterator);
    assertTrue(multiset.isEmpty());
  }

  public void testCopyOf_iterator_oneElement() {
    Iterator<String> iterator = Iterators.singletonIterator("a");
    Multiset<String> multiset = ImmutableSortedMultiset.copyOf(iterator);
    assertEquals(HashMultiset.create(asList("a")), multiset);
  }

  public void testCopyOf_iterator_general() {

    Multiset<String> multiset = ImmutableMultiset.copyOf(iterator);
    assertTrue(multiset.isEmpty());
  }

  public void testCopyOf_iterator_oneElement() {
    Iterator<String> iterator = Iterators.singletonIterator("a");
    Multiset<String> multiset = ImmutableMultiset.copyOf(iterator);
    assertEquals(HashMultiset.create(asList("a")), multiset);
  }

  public void testCopyOf_iterator_general() {

    Multiset<String> multiset = ImmutableSortedMultiset.copyOf(iterator);
    assertTrue(multiset.isEmpty());
  }

  public void testCopyOf_iterator_oneElement() {
    Iterator<String> iterator = Iterators.singletonIterator("a");
    Multiset<String> multiset = ImmutableSortedMultiset.copyOf(iterator);
    assertEquals(HashMultiset.create(asList("a")), multiset);
  }

  public void testCopyOf_iterator_general() {

    List<String> list = ImmutableList.copyOf(iterator);
    assertEquals(Collections.emptyList(), list);
  }

  public void testCopyOf_iterator_oneElement() {
    Iterator<String> iterator = Iterators.singletonIterator("a");
    List<String> list = ImmutableList.copyOf(iterator);
    assertEquals(Collections.singletonList("a"), list);
  }

  public void testCopyOf_iterator_general() {