title: NLTK tree data structure, finding a node, it's parent or children
body: <p>I am using nltk's Tree data structure to work with parsetree strings.</p>
  
  <pre><code>from nltk.tree import Tree
  parsed = Tree('(ROOT (S (NP (PRP It)) (VP (VBZ is) (ADJP (RB so) (JJ nice))) (. .)))')
  </code></pre>
  
  <p>The data structure, however, seems to be limited.
  Is it possible to get a node by it's string value and then navigate to top or bottom?</p>
  
  <p>For example suppose you want to get the node with string value <em>'nice'</em> and then see what's its parent, children, etc. Is it achievable via nltk's Tree? </p>