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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 list_units(self):
 """Return the current list of the Units in the fleet cluster Yields: Unit: The next Unit in the cluster Raises: fleet.v1.errors.APIError: Fleet returned a response code >= 400 """

        for page in self._request('Units.List'):
            for unit in page.get('units', []):
                yield Unit(client=self, data=unit)