public function exportFieldValue($name, $exportHandler = null)
    {
        if (!is_string($name)) {
            throw new InvalidArgumentException(
                "invalid value provided for 'name'; ".
                "expecting a field name as string"
            );
        }
        else if (($field = $this->getField($name)) == null) {
            throw new UnknownFieldException($name);
        }
        return $field->exportValue($exportHandler);
    }