public void start() {
    if (running)
      throw new IllegalStateException();
    start = ticks.ticks();
    running = true;
  }
@Test
  void twiceStarted() {
    assertThrows(IllegalStateException.class, () -> {
      FakeTicks f = new FakeTicks();
      Stopwatch s = new Stopwatch(f);
      s.start();
      s.start();
    });
  }