}.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());
  }

   * <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.

   */
  public static <T extends @Nullable Object> UnmodifiableIterator<T> singletonIterator(
      @ParametricNullness T value) {
    return new SingletonIterator<>(value);
  }

  private static final class SingletonIterator<T extends @Nullable Object>
      extends UnmodifiableIterator<T> {
    private static final Object SENTINEL = new Object();