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

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