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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 _store_timestamp(self, timestamp):
 """If not already saved, cache the first timestamp in the active trace file on the instance. """

        if getattr(self, 'first_timestamp', None) is None:
            self.first_timestamp = timestamp
            LOG.debug("Storing %d as the first timestamp of the trace file %s",
                    self.first_timestamp, self.filename)