public function process()
    {
        $expr = $this->getExpression();
        $state = $this->getState();
        $identifier=  $expr->getIdentifier();

        $var = $state->getVariable($identifier);

        if (is_null($var)) {
            return null;
        } elseif ($var instanceof ResponseVariable) {
            return $var->getCorrectResponse();
        } else {
            $msg = "The variable with identifier '${identifier}' is not a ResponseVariable object.";
            throw new ExpressionProcessingException($msg, $this, ExpressionProcessingException::WRONG_VARIABLE_TYPE);
        }
    }