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.

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