public function addHeader($type = PHPRtfLite_Container_Header::TYPE_ALL, PHPRtfLite_Container_Header $header = null)
    {
        if ($header === null) {
            $header = new PHPRtfLite_Container_Header($this, $type);
        }
        $this->_headers[$type] = $header;

        return $header;
    }