Return the value of the current token and read the next one into
            self.cur_token.

def _advance(self):
        """ Return the value of the current token and read the next one into
            self.cur_token.
        """
        cur_val = None if self.cur_token is None else self.cur_token.value
        try:
            self.cur_token = next(self._tokenizer)
        except StopIteration:
            self.cur_token = None
        return cur_val