public static Sessions withGapDuration(Duration gapDuration) {
    return new Sessions(gapDuration);
  }
@Test
  public void testValidOutputAtEndTimes() throws Exception {
    WindowFnTestUtils.validateGetOutputTimestamps(
        Sessions.withGapDuration(Duration.millis(10)),
        TimestampCombiner.END_OF_WINDOW,
        ImmutableList.of(
            (List<Long>) ImmutableList.of(1L, 3L),
            (List<Long>) ImmutableList.of(0L, 5L, 10L, 15L, 20L)));
  }