use a string -- for now, ``always`` and ``never`` are the options

def set_unlock_previous(self, unlock_previous):
        """use a string -- for now, ``always`` and ``never`` are the options"""
        if unlock_previous is None:
            raise NullArgument('unlock_previous cannot be None')
        if unlock_previous is not None and not utilities.is_string(unlock_previous):
            raise InvalidArgument('unlock_previous must be a string')
        self.my_osid_object_form._my_map['unlockPrevious'] = unlock_previous