@Override
  public ParSeqBasedCompletionStage<Void> thenRun(Runnable action)
  {
    return nextStageByComposingTask(_task.flatMap("thenRun", (t) -> Task.action(action::run)));
  }
@Test
  public void testThenRun() throws Exception
  {
    CompletionStage<String> completionStage = createTestStage(TESTVALUE1);
    Runnable runnable = mock(Runnable.class);
    finish(completionStage.thenRun(runnable));
    verify(runnable, times(1)).run();
  }