code: def _add_utterance_to_document(self, utterance):
          
          utter_id = 'utterance_{}'.format(utterance.attrib['nrgen'])
          norm, lemma, pos = [elem.text.split()
                              for elem in utterance.iterchildren()]
          for i, word in enumerate(utterance.text.split()):
              token_id = self._add_token_to_document(
                  word, token_attrs={self.ns+':norm': norm[i],
                                     self.ns+':lemma': lemma[i],
                                     self.ns+':pos': pos[i]})
              self._add_spanning_relation(utter_id, token_id)
          self.utterances.append(utter_id)
docs: add an utterance to this docgraph (as a spanning relation)