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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 pause(self):
 """Pause the stopwatch. If the stopwatch is already paused, nothing will happen. """

        if self._end_time is not None:
            # the stopwatch is already paused
            return
        self._end_time = datetime.datetime.now()
        self._elapsed_time += self._end_time - self._start_time