public static <T> PCollections<T> pCollections() {
    return new PCollections<>();
  }
@Test
  @Category(ValidatesRunner.class)
  public void testFlattenPCollectionsThenParDo() {
    List<List<String>> inputs = Arrays.asList(LINES, NO_LINES, LINES2, NO_LINES, LINES, NO_LINES);

    PCollection<String> output =
        makePCollectionListOfStrings(p, inputs)
            .apply(Flatten.pCollections())
            .apply(ParDo.of(new IdentityFn<>()));

    PAssert.that(output).containsInAnyOrder(flattenLists(inputs));
    p.run();
  }