Get the preferred subtag.

        :return: preferred :class:`language_tags.Subtag.Subtag` if exists, otherwise None.

def preferred(self):
        """
        Get the preferred subtag.

        :return: preferred :class:`language_tags.Subtag.Subtag` if exists, otherwise None.
        """
        if 'Preferred-Value' in self.data['record']:
            preferred = self.data['record']['Preferred-Value']
            type = self.data['type']
            if type == 'extlang':
                type = 'language'
            return Subtag(preferred, type)
        return None