public static function reveal_identities($assignmentid) {
        global $CFG, $USER;

        $params = self::validate_parameters(self::reveal_identities_parameters(),
                                            array('assignmentid' => $assignmentid));

        list($assignment, $course, $cm, $context) = self::validate_assign($params['assignmentid']);

        $warnings = array();
        if (!$assignment->reveal_identities()) {
            $detail = 'User id: ' . $USER->id . ', Assignment id: ' . $params['assignmentid'];
            $warnings[] = self::generate_warning($params['assignmentid'],
                                                 'couldnotrevealidentities',
                                                 $detail);
        }

        return $warnings;
    }