public function setServiceUrl ($url)
    {
        if ($this->hasBeenOpened()) {
            throw new CAS_OutOfSequenceException(
                'Cannot set the URL, stream already opened.'
            );
        }
        if (!is_string($url) || !strlen($url)) {
            throw new CAS_InvalidArgumentException('Invalid url.');
        }

        $this->_url = $url;
    }