public function setSectionIdentifier($sectionIdentifier)
    {
        if (Format::isIdentifier($sectionIdentifier) || empty($sectionIdentifier)) {
            $this->sectionIdentifier = $sectionIdentifier;
        } else {
            $msg = "'${sectionIndentifier}' is not a valid QTI Identifier.";
            throw new InvalidArgumentException($msg);
        }
    }