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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 _MergeSameAgency(self, a_agency_id, b_agency_id):
 """Merge agency ids to the corresponding agency id in the merged schedule. Args: a_agency_id: an agency id from the old schedule b_agency_id: an agency id from the new schedule Returns: The agency id of the corresponding merged agency. Raises: MergeError: If a_agency_id and b_agency_id do not correspond to the same merged agency. KeyError: Either aaid or baid is not a valid agency id. """

    a_agency_id = (a_agency_id or
                   self.feed_merger.a_schedule.GetDefaultAgency().agency_id)
    b_agency_id = (b_agency_id or
                   self.feed_merger.b_schedule.GetDefaultAgency().agency_id)
    a_agency = self.feed_merger.a_schedule.GetAgency(
        a_agency_id)._migrated_entity
    b_agency = self.feed_merger.b_schedule.GetAgency(
        b_agency_id)._migrated_entity
    if a_agency != b_agency:
      raise MergeError('agency must be the same')
    return a_agency.agency_id