rdf.pl into a list of rdf/3
triples. The identifier of the main object created is returned
by rdf_triples/3.
Input is the `content' of the RDF element in the format as
generated by load_structure(File, Term, [dialect(xmlns)]).
rdf_triples/3 can process both individual descriptions as
well as the entire content-list of an RDF element. The first
mode is suitable when using library(sgml) in `call-back' mode.