docs: def phrase_strings(self, phrase_type):
          """
          Returns strings 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 strings representing those phrases
  
          """
          return [u" ".join(subtree.leaves()) for subtree in self.subtrees_for_phrase(phrase_type)]