def count_sents(self, sents, verbose=False):
        """
            sents : a list of sentences, each a list of tokenized symbols
        """
        if verbose:
            logger.info("counting {} sents ...".format(len(sents)))
        for idx, symbols in enumerate(sents):
            if verbose and idx > 0 and idx % 500000 == 0:
                logger.info("    line {}".format(idx))
            self.counter.update(symbols)

            sents : a list of sentences, each a list of tokenized symbols
        
xtune/src/transformers/tokenization_transfo_xl.py