public function setUrl($url)
    {
        if ($this->hasBeenSent()) {
            throw new CAS_OutOfSequenceException(
                'Cannot set the URL, request already sent.'
            );
        }
        if (!is_string($url)) {
            throw new CAS_InvalidArgumentException('$url must be a string.');
        }

        $this->_url = $url;
    }