def GetDefaultAgency(self):
    """Return the default Agency. If no default Agency has been set select the
    default depending on how many Agency objects are in the Schedule. If there
    are 0 make a new Agency the default, if there is 1 it becomes the default,
    if there is more than 1 then return None.
    """
    if not self._default_agency:
      if len(self._agencies) == 0:
        self.NewDefaultAgency()
      elif len(self._agencies) == 1:
        self._default_agency = self._agencies.values()[0]
    return self._default_agency
Return the default Agency. If no default Agency has been set select the
    default depending on how many Agency objects are in the Schedule. If there
    are 0 make a new Agency the default, if there is 1 it becomes the default,
    if there is more than 1 then return None.