docs: def __add_token_annotation_tier(self, tier):
          """
          adds a tier to the document graph, in which each event annotates
          exactly one token.
          """
          for i, event in enumerate(tier.iter('event')):
              anno_key = '{0}:{1}'.format(self.ns, tier.attrib['category'])
              anno_val = event.text if event.text else ''
              self.node[event.attrib['start']][anno_key] = anno_val