public function addVariable(Variable $variable) {
        $output = &$this->getOutput();
        $varName = $variable->getIdentifier();
        $marshaller = new taoQtiCommon_helpers_PciJsonMarshaller();
        $output[$varName] = $marshaller->marshall($variable->getValue(), taoQtiCommon_helpers_PciJsonMarshaller::MARSHALL_ARRAY);
    }