public void stop() {
    if (!running)
      throw new IllegalStateException();
    elapsed = ticks.ticks() - start;
    running = false;
  }
@Test
  void notStarted2() {
    assertThrows(IllegalStateException.class, () -> {
      FakeTicks f = new FakeTicks();
      Stopwatch s = new Stopwatch(f);
      s.stop();
    });
  }