Adding graphs to an in-memory dataset

[Cindy A McMullen]

How do I add a named graph to an in-memory Dataset?  This code works with TDB, 
but the in-memory implementation throws:

Exception in thread "main" com.hp.hpl.jena.shared.JenaException: No such graph: 
ns:Event1
at 
com.hp.hpl.jena.sparql.core.DatasetGraphCollection.add(DatasetGraphCollection.java:41)
     [java]     at 
oracle.social.discovery.rdf.jena.QueryMemoryGraph.addData(QueryMemoryGraph.java:94)
     [java]     at 
oracle.social.discovery.rdf.jena.QueryMemoryGraph.main(QueryMemoryGraph.java:33)

Here's the code:

String inputFileName  = "foo-ontology.ttl";
        Model model = ModelFactory.createDefaultModel();
        InputStream in = FileManager.get().open( inputFileName );
        model.read(in, "", "TTL");
        DatasetGraph dsg =  DatasetGraphFactory.create(model.getGraph());
……

Node g1 = Node.createURI("ns:Event1");
        Triple e1 = new Triple(Node.createURI("w:Mike"), 
Node.createURI("w:follows"), Node.createURI("w:Dave"));
        dsg.add(new Quad(g1, e1));

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I'm using apache-jena-2.10.0