public function setProxyTicket ($proxyTicket)
    {
        if (empty($proxyTicket)) {
            throw new CAS_InvalidArgumentException(
                'Trying to initialize with an empty proxy ticket.'
            );
        }
        if (!empty($this->_proxyTicket)) {
            throw new CAS_OutOfSequenceException(
                'Already initialized, cannot change the proxy ticket.'
            );
        }
        $this->_proxyTicket = $proxyTicket;
    }