title: looking for context of words inside corpus categories doesn't work
body: <p>I wrote this little script to find the context of the 10 most frequent words in my corpus. But it doesn't work and I have no idea what I'm doing wrong.The tien_frequentste(mijn_corpus) definition works on it's own.</p>
  
  <pre><code>tienfrequentste = tien_frequentste(mijncorpus)
  def context (corpus, most_freq):
     for category in corpus.categories():
       print "Context voor" , category, ":"
         for word in most_freq:
             print nltk.Text(corpus.words(categories=category)).concordance(word)
  </code></pre>
  
  <p>UPDATE:<br>
  I get an error message on traceback<br>
  for <code>context(corpus, most_freq)</code>,<br>
  for <code>category in corpus.categories()</code>,<br>
  for <code>self.init()</code><br>
  and <code>in_init</code>. 
  And an <code>AttributeError:'NoneType' object has no attribute 'group'</code>.<br>
  No idea what these errors mean..</p>
  
  <pre><code>Traceback (most recent call last):
   File "/Users/...document.py", line 92, in &lt;module&gt; context (mijn_corpus, tienfrequentste)
  
   File "/Users/...document.py", line 87, in context for category in corpus.categories(): File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/nltk.corpus.reader.api.py, line 317, in categories self.init().
  
  File "/Users/...document.py", line 87, in context for category in corpus.categories(): File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/nltk.corpus.reader.api.py, line 289, in_init category = re.match(self._pattern, file id).group(1)
  
  attributeError: 'Nonetype' object has no attribute "group"
  </code></pre>