returns True, iff all events in the given tier annotate exactly one
        token.

def is_token_annotation_tier(self, tier):
        """
        returns True, iff all events in the given tier annotate exactly one
        token.
        """
        for i, event in enumerate(tier.iter('event')):
            if self.indexdelta(event.attrib['end'], event.attrib['start']) != 1:
                return False
        return True