protected function parseExpressionPart($part, NumberSystem $sourceSystem, NumberSystem $targetSystem)
    {
        try {
            // if it is a valid number of the source-system convert it
            $sourceNumber = new Number($part, $sourceSystem);

            return $sourceNumber->convert($targetSystem)->value();
        } catch (NumberParseException $e) {
            return $part;
        }
    }