public function open()
    {
        if ($this->_handle !== null) {
            return;
        }

        $this->_handle = fopen($this->_filename, 'wr', false);
        if (!$this->_handle) {
            throw new PHPRtfLite_Exception("Could not open rtf output stream (url: $this->_filename)!");
        }
        flock($this->_handle, LOCK_EX);
    }