docs: 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