public function addRequest (CAS_Request_RequestInterface $request)
    {
        if ($this->_sent) {
            throw new CAS_OutOfSequenceException(
                'Request has already been sent cannot '.__METHOD__
            );
        }
        if (!$request instanceof CAS_Request_CurlRequest) {
            throw new CAS_InvalidArgumentException(
                'As a CAS_Request_CurlMultiRequest, I can only work with CAS_Request_CurlRequest objects.'
            );
        }

        $this->_requests[] = $request;
    }