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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 _add_trustee(self, device):
        '''Add a single trusted device to the trust domain.

        :param device: ManagementRoot object -- device to add to trust domain
        '''

        device_name = get_device_info(device).name
        if device_name in self.domain:
            msg = 'Device: %r is already in this trust domain.' % device_name
            raise DeviceAlreadyInTrustDomain(msg)
        self._modify_trust(self.devices[0], self._get_add_trustee_cmd, device)