}.test();
  }

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

    }.test();
  }

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

  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;
    }

  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;
    }