public function col_unsubscribe(\tool_monitor\subscription $sub) {
        global $OUTPUT, $CFG;

        $deleteurl = new \moodle_url($CFG->wwwroot. '/admin/tool/monitor/index.php', array('subscriptionid' => $sub->id,
                'action' => 'unsubscribe', 'courseid' => $this->courseid, 'sesskey' => sesskey()));
        $icon = $OUTPUT->render(new \pix_icon('t/delete', get_string('deletesubscription', 'tool_monitor')));

        return \html_writer::link($deleteurl, $icon, array('class' => 'action-icon'));
    }