add an utterance to this docgraph (as a spanning relation)

def _add_utterance_to_document(self, utterance):
        """add an utterance to this docgraph (as a spanning relation)"""
        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)