public function col_select(stdClass $row) {
        $selectcol = '<label class="accesshide" for="selectuser_' . $row->userid . '">';
        $selectcol .= get_string('selectuser', 'assign', $this->assignment->fullname($row));
        $selectcol .= '</label>';
        $selectcol .= '<input type="checkbox"
                              id="selectuser_' . $row->userid . '"
                              name="selectedusers"
                              value="' . $row->userid . '"/>';
        $selectcol .= '<input type="hidden"
                              name="grademodified_' . $row->userid . '"
                              value="' . $row->timemarked . '"/>';
        $selectcol .= '<input type="hidden"
                              name="gradeattempt_' . $row->userid . '"
                              value="' . $row->attemptnumber . '"/>';
        return $selectcol;
    }