public static <T> PCollections<T> pCollections() {
    return new PCollections<>();
  }
@Test
  @Category(ValidatesRunner.class)
  public void testFlattenPCollectionsSingletonList() {
    PCollection<String> input = p.apply(Create.of(LINES));
    PCollection<String> output = PCollectionList.of(input).apply(Flatten.pCollections());

    assertThat(output, not(equalTo(input)));

    PAssert.that(output).containsInAnyOrder(LINES);
    p.run();
  }