public function add($key, $default, $type, $allow_null) {
        $obj = new stdclass();
        $obj->type = is_int($type) ? $type : HTMLPurifier_VarParser::$types[$type];
        if ($allow_null) $obj->allow_null = true;
        $this->info[$key] = $obj;
        $this->defaults[$key] = $default;
        $this->defaultPlist->set($key, $default);
    }