def set_unlock_previous(self, unlock_previous):
if unlock_previous is None:<EOL><INDENT>raise NullArgument('<STR_LIT>')<EOL><DEDENT>if unlock_previous is not None and not utilities.is_string(unlock_previous):<EOL><INDENT>raise InvalidArgument('<STR_LIT>')<EOL><DEDENT>self.my_osid_object_form._my_map['<STR_LIT>'] = unlock_previous<EOL>
use a string -- for now, ``always`` and ``never`` are the options