title: Add conjunction to grammar rule - NLTK parse into syntax tree in python
body: <p>Suppose  we  have following incomplete  grammar rule:</p>
  
  <pre><code>grammar2 = nltk.parse_cfg("""
  S -&gt; NP VP
  NP -&gt; Det N
  VP -&gt; V NP
  PN -&gt; 'David' 
  Det -&gt; 'the'
  N -&gt; 'man' 
  V -&gt; 'saw' | 'helped' 
  Pro -&gt; 'him'
  Conj -&gt; 'and'
  """)
  </code></pre>
  
  <p>I want to create syntax tree for following sentence:</p>
  
  <pre><code>sent = ['the', 'man', 'saw', 'David', 'and', 'helped', 'him']
  parser = nltk.ChartParser(grammar2)
  trees = parser.nbest_parse(sent)
  for tree in trees:
      print tree
  </code></pre>
  
  <p>But I don't know how can I add conjunction <code>and</code> in the grammar?</p>