}.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 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 final T value;
    private boolean done;

    SingletonIterator(T value) {
      this.value = value;
    }