import nltkfrom nltk.corpus import brownbrown_tagged_sents = brown.tagged_sents(categories='news')((x[1],y[1],z[0],z[1])    for sent in brown_tagged_sents    for x,y,z in nltk.trigrams(sent))cfd = nltk.ConditionalFreqDist(    ((x[1],y[1],z[0],z[1])    for sent in brown_tagged_sents    for x,y,z in nltk.trigrams(sent)))會出現上面的代碼：Traceback (most recent call last):    File "/Users/shike/Desktop/textprocess/nltktest.py", line 10, in <module>      for sent in brown_tagged_sents    File "/usr/local/lib/python3.6/site-packages/nltk/probability.py", line   1803, in __init__      for (cond, sample) in cond_samples:  ValueError: too many values to unpack (expected 2)  該列表理解((x[1],y[1],z[0],z[1])    for sent in brown_tagged_sents    for x,y,z in nltk.trigrams(sent))可以單獨工作，但是當它用作參數時會出錯。我仍然不知道為什么。所有代碼均在 Python3 中測試。