docs: 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