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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 ResolveCronJobFlowURN(self, cron_job_id):
 """Resolve a URN of a flow with this id belonging to a given cron job."""

    if not self._value:
      raise ValueError("Can't call ResolveCronJobFlowURN on an empty "
                       "client id.")

    return cron_job_id.ToURN().Add(self._value)