Attempting to install the relevant ISO 2- and 3-letter 
         codes as the enumerated possible values is probably never 
         going to be a realistic possibility.  See 
         RFC 3066 at http://www.ietf.org/rfc/rfc3066.txt and the IANA registry 
         at http://www.iana.org/assignments/lang-tag-apps.htm for 
         further information. 
 
         The union allows for the 'un-declaration' of xml:lang with 
         the empty string. |