public long elapsedNanos() {
    if (running) {
      return this.ticks.ticks() - start;
    } else {
      if (elapsed == -1)
        throw new IllegalStateException();
      return elapsed;
    }
  }
@Test
  void notStarted1() {
    assertThrows(IllegalStateException.class, () -> {
      FakeTicks f = new FakeTicks();
      Stopwatch s = new Stopwatch(f);
      s.elapsedNanos();
    });
  }