NLTK:如何访问分块字符串

import nltk
from nltk.corpus import treebank_chunk

grammar = r"""
    NP: {<DT|JJ|NN.*>+}          # 名词短语块
    PP: {<IN><NP>}               # 介词短语块
    VP: {<VB.*><NP|PP|CLAUSE>+$} # 动词短语块
    CLAUSE: {<NP><VP>}           # 从句块
"""

chunk_parser = nltk.RegexpParser(grammar)

sent = [("the", "DT"), ("big", "JJ"), ("cat", "NN"), ("chased", "VBD"), ("the", "DT"), ("mouse", "NN")]
result = chunk_parser.parse(sent)

for subtree in result.subtrees():
    if subtree.label() == "NP":
        print(subtree)