adds a tier to the document graph, in which each event annotates
        exactly one token.

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