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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_unit_states(self, machine_id=None, unit_name=None):
 """Return the current UnitState for the fleet cluster Args: machine_id (str):
 filter all UnitState objects to those originating from a specific machine unit_name (str):
 filter all UnitState objects to those related to a specific unit Yields: UnitState: The next UnitState in the cluster Raises: fleet.v1.errors.APIError: Fleet returned a response code >= 400 """

        for page in self._request('UnitState.List', machineID=machine_id, unitName=unit_name):
            for state in page.get('states', []):
                yield UnitState(data=state)