I think it sounds good. I will add that to the docstring.

Also, I just found out there were many places in nltk/tree.py still using old string initilization syntax. e.g. line 212:

">>> t = Tree('(S (NP (D the) (N dog)) (VP (V chased) (NP (D the) (N cat))))')"

Sorry I forget to change these. I will replace them all with Tree.fromstring instead