title: NLTK BigramTagger does not tag half of the sentence
body: <p>Can someone please explain the behaviour of NLTK's BigramTagger in these examples?</p>
  
  <p>I instantiated the tagger by </p>
  
  <pre><code>bi= BigramTagger(brown.tagged_sents(categories='news')[:500])
  </code></pre>
  
  <p>Now, I want to use this on one specific sentence.</p>
  
  <pre><code>&gt;&gt;&gt; bi.tag(brown_sents[2])
  [(u'The', u'AT'), (u'September-October', u'NP'), (u'term', u'NN'), (u'jury', u'NN'), (u'had', u'HVD'), (u'been', u'BEN'), (u'charged', u'VBN'), (u'by', u'IN'), (u'Fulton', u'NP-TL'), (u'Superior', u'JJ-TL'), (u'Court', u'NN-TL'), (u'Judge', u'NN-TL'), (u'Durwood', u'NP'), (u'Pye', u'NP'), (u'to', u'TO'), (u'investigate', u'VB'), (u'reports', u'NNS'), (u'of', u'IN'), (u'possible', u'JJ'), (u'``', u'``'), (u'irregularities', u'NNS'), (u"''", u"''"), (u'in', u'IN'), (u'the', u'AT'), (u'hard-fought', u'JJ'), (u'primary', u'NN'), (u'which', u'WDT'), (u'was', u'BEDZ'), (u'won', u'VBN'), (u'by', u'IN'), (u'Mayor-nominate', u'NN-TL'), (u'Ivan', u'NP'), (u'Allen', u'NP'), (u'Jr.', u'NP'), (u'.', u'.')]
  </code></pre>
  
  <p>Works well, but hey, it's all known data.
  Let me change one word and see if it sets something off.</p>
  
  <pre><code>&gt;&gt;&gt; sent=brown_sents[2]
  &gt;&gt;&gt; sent[5]
  u'been'
  &gt;&gt;&gt; sent[5] = u'was'
  &gt;&gt;&gt; bi.tag(sent)
  [(u'The', u'AT'), (u'September-October', u'NP'), (u'term', u'NN'), (u'jury', u'NN'), (u'had', u'HVD'), (u'was', None), (u'charged', None), (u'by', None), (u'Fulton', None), (u'Superior', None), (u'Court', None), (u'Judge', None), (u'Durwood', None), (u'Pye', None), (u'to', None), (u'investigate', None), (u'reports', None), (u'of', None), (u'possible', None), (u'``', None), (u'irregularities', None), (u"''", None), (u'in', None), (u'the', None), (u'hard-fought', None), (u'primary', None), (u'which', None), (u'was', None), (u'won', None), (u'by', None), (u'Mayor-nominate', None), (u'Ivan', None), (u'Allen', None), (u'Jr.', None), (u'.', None)]
  </code></pre>
  
  <p>Now I expected to see changed tuple, <code>(u'been', u'BEN')</code> to now be (u'been', None). Why is everything after it in the sentence now not tagged? Those words were tagged in connection to another ones, not 'been'.</p>
  
  <p>Any recommendation on use of tagged sentences would be appreciated as well.</p>