}

  public void testOnlyElementSingleton() {
    assertThat(Stream.of(1).collect(onlyElement())).isEqualTo(1);
  }

  public void testOnlyElementNull() {
    assertThat(Stream.<@Nullable Object>of((Object) null).collect(onlyElement())).isNull();
  }

  public void testOnlyElementMultiple() {
    IllegalArgumentException expected =