def _keys_from_label(self, label):
        '''Convert a label into a kvl key.
        '''
        k1 = (label.content_id1, label.content_id2,
              label.annotator_id, time_complement(label.epoch_ticks))
        k2 = (label.content_id2, label.content_id1,
              label.annotator_id, time_complement(label.epoch_ticks))
        return k1, k2
Convert a label into a kvl key.