docs: def __add_tokens(self, token_tier):
          """
          adds all tokens to the document graph. Exmaralda considers them to
          be annotations as well, that's why we could only extract the token
          node IDs from the timeline (using ``__add_tokenization()``), but not
          the tokens themselves.
  
          Parameters
          ----------
          token_tier : etree._Element
              an etree element representing the <tier> which contains the tokens
          """
          for event in token_tier.iter('event'):
              assert len(self.gen_token_range(event.attrib['start'],
                         event.attrib['end'])) == 1, \
                  "Events in the token tier must not span more than one token."
              token_id = event.attrib['start']
              self.node[token_id][self.ns+':token'] = event.text