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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 SetDefaultAgency(self, agency, validate=True):
 """Make agency the default and add it to the schedule if not already added"""

    assert isinstance(agency, self._gtfs_factory.Agency)
    self._default_agency = agency
    if agency.agency_id not in self._agencies:
      self.AddAgencyObject(agency, validate=validate)