code: def stop(self):
          
          if self._start_instant is None:
              raise IntervalException("Attempt to stop an interval that has not started.")
          if self._stop_instant is None:
              self._stop_instant = instant()
              self._duration = int((self._stop_instant - self._start_instant) * 1000)
              return self._duration
          return False
docs: Mark the stop of the interval.
  
          Calling stop on an already stopped interval has no effect.
          An interval can only be stopped once.
  
          :returns: the duration if the interval is truely stopped otherwise ``False``.