protected function index_submissions_by_authors($submissions) {
        $byauthor = array();
        if (is_array($submissions)) {
            foreach ($submissions as $submissionid => $submission) {
                if (isset($byauthor[$submission->authorid])) {
                    throw new moodle_exception('moresubmissionsbyauthor', 'workshop');
                }
                $byauthor[$submission->authorid] = $submission;
            }
        }
        return $byauthor;
    }