code: def next(self):
          
          if self.current_token == len(self.tokens):
              return None
          token = self.tokens[self.current_token]
          if token["type"] == "newline":
              self.line += 1
              self.line_start = token["start"]
          self.current_token += 1
          if token["type"] == "unknown":
              self.error("Unknown token")
          return token
docs: Return the next token.  Keep track of our current position in the
          config for nice errors.