NLTK was unable to find the gs file
<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>