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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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