title: How to navigate a nltk.tree.Tree?
body: <p>I've chunked a sentence using:</p>
  
  <pre><code>grammar = '''                                                                                                              
      NP:                                                                                                                    
         {&lt;DT&gt;*(&lt;NN.*&gt;|&lt;JJ.*&gt;)*&lt;NN.*&gt;}                                                                                       
       NVN:                                                                                                                  
         {&lt;NP&gt;&lt;VB.*&gt;&lt;NP&gt;}                                                                                                    
      '''
  chunker = nltk.chunk.RegexpParser(grammar)
  tree = chunker.parse(tagged)
  print tree
  </code></pre>
  
  <p>The result looks like:</p>
  
  <pre><code>(S
    (NVN
      (NP The_Pigs/NNS)
      are/VBP
      (NP a/DT Bristol-based/JJ punk/NN rock/NN band/NN))
    that/WDT
    formed/VBN
    in/IN
    1977/CD
    ./.)
  </code></pre>
  
  <p>But now I'm stuck trying to figure out how to navigate that.  I want to be able to find the NVN subtree, and access the left-side noun phrase ("The_Pigs"), the verb ("are") and the right-side noun phrase ("a Bristol-based punk rock band").  How do I do that?</p>