protected function _executeCallbackStack()
    {
        if (!empty($this->callback_stack)) {
            $_meth = '_executeCallbackAs'.ucfirst($this->callback_response_type);
            foreach ($this->callback_stack as $_callback) {
                $response = $this->$_meth( $_callback, $this->value );
            }
            return $response;
        }
        return null;
    }