def get_last_tid(self):
        try:
            spec = {'tid': {'$gt': 0}}
            sort = [('tid', -1)]
            row = self.db.docs.find_one(spec, sort=sort)
        except Exception as e:
            raise DatabaseException(e)
        if row:
            return row['tid']
        return 0
def __add__(self, other):
        assert self.n_classes == other.n_classes, "Number of classes does not match"
        return MulticlassContingencyTable(self.table + other.table,
                                          n_classes=self.n_classes,
                                          class_names=self.class_names)