How can we get trees in the scene?
There is a method for iterating over NodeDataModels (iterateOverNodeDataDependentOrder) but this function iterates over all nodes, I want a method to iterate over each tree, and it must see each node toward flow direction.
I've written a subclass of NodeDataModel with virtual method process, to add a process to each node. I want to separate each tree and run them in separate threads.

class RunnableDataModel: public NodeDataModel
{
  virtual void preprocess();
  virtual int process() = 0;
  virtual void postprocess();
...
}