code: def subtrees_for_phrase(self, phrase_type):
          
          return [subtree for subtree in self.parse.subtrees() if subtree.node.lower() == phrase_type.lower()]
docs: Returns subtrees corresponding all phrases matching a given phrase type
  
          :param phrase_type: POS such as "NP", "VP", "det", etc.
          :type phrase_type: str
  
          :return: a list of NLTK.Tree.Subtree instances
          :rtype: list of NLTK.Tree.Subtree