public function add_user_context(int $userid) : contextlist {
        $sql = "SELECT DISTINCT ctx.id
                  FROM {context} ctx
                 WHERE ctx.contextlevel = :contextlevel
                   AND ctx.instanceid = :instanceid";
        return $this->add_from_sql($sql, ['contextlevel' => CONTEXT_USER, 'instanceid' => $userid]);
    }