docs: def AddArg(self, arg):
      """Adds a new arg to this expression.
  
      Args:
         arg: The argument to add (string).
  
      Returns:
        True if this arg is the last arg, False otherwise.
  
      Raises:
        ParseError: If there are too many args.
      """
      self.args.append(arg)
      if len(self.args) > self.number_of_args:
        raise ParseError("Too many args for this expression.")
  
      elif len(self.args) == self.number_of_args:
        return True
  
      return False