@Before
 public void createGraph() {
   graph = new SimpleMGraph();
   graph.add(TripleUtil.uriTriple("S1", "P1", "01"));
   graph.add(TripleUtil.uriTriple("S1", "P1", "02"));
   graph.add(TripleUtil.uriTriple("S2", "P1", "01"));
   graph.add(TripleUtil.uriTriple("S2", "P1", "02"));
   graph.add(TripleUtil.uriTriple("S3", "P1", "01"));
   graph.add(TripleUtil.uriTriple("S4", "P1", "02"));
 }