public function setOutputFormat($outputFormat)
    {
        $allowedFormats = array(
            'xml/rdf',
            'text/simple',
            'text/microformats',
            'application/json',
            'text/n3'
        );

        if (!in_array(strtolower($outputFormat), $allowedFormats)) {
            throw new \InvalidArgumentException(
                "Illegal output format, allowed output formats are " . implode(', ', $allowedFormats)
            );
        }
        $this->outputFormat = $outputFormat;

        return $this;
    }