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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 bll_version(self):
 """Get the BLL version this session is connected to. Return: Version string if session started. None if session not started. """

        if not self.started():
            return None
        status, data = self._rest.get_request('objects', 'system1',
                                              ['version', 'name'])
        return data['version']