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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 unregister(self, alias):
 """Unregisters a service instance. Stops a service and removes it from the manager. Args: alias: string, the alias of the service instance to unregister. """

        if alias not in self._service_objects:
            raise Error(self._device,
                        'No service is registered with alias "%s".' % alias)
        service_obj = self._service_objects.pop(alias)
        if service_obj.is_alive:
            with expects.expect_no_raises(
                    'Failed to stop service instance "%s".' % alias):
                service_obj.stop()