SmartGWT: how to make nested form

DynamicForm form = new DynamicForm();
CanvasItem canvasItem = new CanvasItem();
canvasItem.setColSpan(2);
DynamicForm subForm = new DynamicForm();
... ...
subForm.setItems(...);
canvasItem.setCanvas(subForm);
... ...
form.setItems(canvasItem, ...);