rdf(S,P,O) terms 
extracted from DOM according to the RDFa specification. Options 
processed:
- base(+BaseURI)
 - URI to use for ''. Normally set to the document URI.
 - anon_prefix(+AnnonPrefix)
 - Prefix for blank nodes.
 - lang(+Lang)
 - Default for 
lang - vocab(+Vocab)
 - Default for 
vocab - markup(+Markup)
 - Markup language processed (xhtml, xml, ...)