@Override
public void undo() {
Iterator<Graph> it = parents.iterator();
for (Object obj : list) {
if (obj instanceof VertexEditPart) {
VertexEditPart part = (VertexEditPart) obj;
Vertex vertex = (Vertex) part.getModel();
Graph parent = it.next();
parent.addVertex(vertex);
// update bounds
Rectangle bounds = (Rectangle) vertex