title: NLTK was unable to find the gs file
body: <p>I'm trying to use NLTK, the stanford natural language toolkit.
  After install the required files, I start to execute the demo code:
  <a href="http://www.nltk.org/index.html" rel="noreferrer">http://www.nltk.org/index.html</a></p>
  
  <pre><code>&gt;&gt;&gt; import nltk
  
  &gt;&gt;&gt; sentence = """At eight o'clock on Thursday morning
  ... Arthur didn't feel very good."""
  
  &gt;&gt;&gt; tokens = nltk.word_tokenize(sentence)
  
  &gt;&gt;&gt; tokens
  
  ['At', 'eight', "o'clock", 'on', 'Thursday', 'morning',
  </code></pre>
  
  <p>'Arthur', 'did', "n't", 'feel', 'very', 'good', '.']</p>
  
  <pre><code>&gt;&gt;&gt; tagged = nltk.pos_tag(tokens)
  
  &gt;&gt;&gt; tagged[0:6]
  
  [('At', 'IN'), ('eight', 'CD'), ("o'clock", 'JJ'), ('on', 'IN'),
  </code></pre>
  
  <p>('Thursday', 'NNP'), ('morning', 'NN')]</p>
  
  <pre><code>&gt;&gt;&gt; entities = nltk.chunk.ne_chunk(tagged)
  
  &gt;&gt;&gt; entities
  </code></pre>
  
  <p>Then I get message:</p>
  
  <pre><code>LookupError: 
  
  ===========================================================================
  NLTK was unable to find the gs file!
  Use software specific configuration paramaters or set the PATH environment variable.
  </code></pre>
  
  <p>I tried google, but there's no one tell what the missing gs file is.</p>