code: def _forward(self):
          
          try:
              self.current_token = next(self.tokens)
          except StopIteration:
              raise MissingTokensError("Unexpected end of token stream at %d." %
                  self.current_pos)
          self.current_pos += 1
docs: Advance to the next token.
  
          Internal methods, updates:
          - self.current_token
          - self.current_pos
  
          Raises:
              MissingTokensError: when trying to advance beyond the end of the
                  token flow.