code: def _tokens_to_subtoken(self, tokens):
          
          ret = []
          for token in tokens:
              ret.extend(
                  self._escaped_token_to_subtoken_strings(_escape_token(token, self._alphabet)))
          return ret
docs: Converts a list of tokens to a list of subtoken.
  
          Args:
            tokens: a list of strings.
          Returns:
            a list of integers in the range [0, vocab_size)