assertThat(itr2.next()).isEqualTo("c");
    assertThat(itr2.next()).isEqualTo("d");
  }

  /** Illustrates the somewhat bizarre behavior when a null is passed in. */
  public void testConcatContainingNull() {
    Iterator<Iterator<Integer>> input =
        (Iterator<Iterator<Integer>>)
            Arrays.<@Nullable Iterator<Integer>>asList(iterateOver(1, 2), null, iterateOver(3))
                .iterator();

    assertThat(itr2.next()).isEqualTo("c");
    assertThat(itr2.next()).isEqualTo("d");
  }

  /** Illustrates the somewhat bizarre behavior when a null is passed in. */
  public void testConcatContainingNull() {
    Iterator<Iterator<Integer>> input =
        (Iterator<Iterator<Integer>>)
            Arrays.<@Nullable Iterator<Integer>>asList(iterateOver(1, 2), null, iterateOver(3))
                .iterator();