/** Creates a graph with a single accept-vertex. */
public static DirectedSparseGraph initialise(){
DirectedSparseGraph pta = new DirectedSparseGraph();
DirectedSparseVertex init = new DirectedSparseVertex();
init.addUserDatum(JUConstants.INITIAL, true, UserData.SHARED);
init.addUserDatum(JUConstants.ACCEPTED, true, UserData.SHARED);
pta.setUserDatum(JUConstants.TITLE, "Hypothesis machine", UserData.SHARED);
pta.addVertex(init);
DeterministicDirectedSparseGraph.numberVertices(pta);
return pta;