How to open "file://"-RDF, change it and flush it to somewhere else?

Hello,

what do I have to do to open a "file://"-RDF and save it, after 
changing, to somewhere else *without* changing the initially opened 
file?

Currently I use this code:

| var rdfService = Components.classes["@mozilla.org/rdf/rdf-service;1"].
|                    getService(Components.interfaces.nsIRDFService);
| 
| var ds=rdfService.GetDataSourceBlocking("file:///tmp/inputtest.rdf");
| 
| var subject = rdfService.GetResource("http://www.example.com/test");
| var predicate = 
|      rdfService.GetResource("http://www.example.com/rdf#name");
| var name = rdfService.GetLiteral("This is a test");
| 
| ds.Assert(subject, predicate, name, true);
| 
| ds.QueryInterface(Components.interfaces.nsIRDFRemoteDataSource);
| ds.FlushTo("file:///tmp/outtest.rdf");
| rdfService.UnregisterDataSource(ds);

This saves my output file, but always changes the input file, too.

Thank you very much in advance

CU

Manuel