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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def stop(self, timeout_s=None):
 """Stops execution of the current phase, if any. It will raise a ThreadTerminationError, which will cause the test to stop executing and terminate with an ERROR state. Args: timeout_s: int or None, timeout in seconds to wait for the phase to stop. """

    self._stopping.set()
    with self._current_phase_thread_lock:
      phase_thread = self._current_phase_thread
      if not phase_thread:
        return

    if phase_thread.is_alive():
      phase_thread.kill()

      _LOG.debug('Waiting for cancelled phase to exit: %s', phase_thread)
      timeout = timeouts.PolledTimeout.from_seconds(timeout_s)
      while phase_thread.is_alive() and not timeout.has_expired():
        time.sleep(0.1)
      _LOG.debug('Cancelled phase %s exit',
                 "didn't" if phase_thread.is_alive() else 'did')
    # Clear the currently running phase, whether it finished or timed out.
    self.test_state.stop_running_phase()