function helper($helper_name) {
        if(array_key_exists($helper_name, $this->helpers)) {
            return $this->helpers[$helper_name];
        }
        else {
            $classname = 'Helper_' . ucwords($helper_name);
            return new $classname($this->modules, $this->server, $this->request, $this->security);
        }
    }