RDF.hs: added a couple more query functions to Graph.
RDF.hs: added a couple more query functions to Graph.

darcs-hash:20070930081637-db1bb-b18f78698ee07601e068ea0f46c582f915750b63

cordawyn/rdf4h,jutaro/rdf4h,LeifW/rdf4h,ddssff/rdf4h