code: def _get_sentences_dict(self):
          
          if self._sentences_dict is None:
              sentences = [Sentence(element) for element in self._xml.xpath('/root/document/sentences/sentence')]
              self._sentences_dict = OrderedDict([(s.id, s) for s in sentences])
          return self._sentences_dict
docs: Returns sentence objects
  
          :return: order dict of sentences
          :rtype: collections.OrderedDict