protected function processOptions($filename = null)
    {
        // output must be first option after operation
        if ($filename!==null) {
            $this->addArg('output', $filename, true);
        }
        foreach ($this->_options as $option) {
            if (is_array($option)) {
                $this->addArg($option[0], $option[1], $option[2]);
            } else {
                $this->addArg($option);
            }
        }
    }