Map id(s) to word(s)

        Parameters
        ----------
        xs : int
            id or a list of ids

        Returns
        -------
        str or list
            word or a list of words

def id2word(self, xs):
        """Map id(s) to word(s)

        Parameters
        ----------
        xs : int
            id or a list of ids

        Returns
        -------
        str or list
            word or a list of words
        """
        if isinstance(xs, list):
            return [self._id2word[x] for x in xs]
        return self._id2word[xs]