model.addNode(this, a, model.getRoot());
model.addNode(this, c, model.getRoot());
model.addNode(this, b, model.getRoot());
model.addNode(this, c2, model.getRoot());
model.addNode(this, d, c);
model.addNode(this, d2, c);
Edge e = model.createEdge("edge");
model.connectEdge(this, e, a, b);
e = model.createEdge("edge");
model.connectEdge(this, e, a, d);