Display help messages or invoke the proper completer.

        The interface of helper methods and completer methods are documented in
        the helper() decorator method and the completer() decorator method,
        respectively.

        Arguments:
            text: A string, that is the current completion scope.
            state: An integer.

        Returns:
            A string used to replace the given text, if any.
            None if no completion candidates are found.

        Raises:
            This method is called via the readline callback. If this method
            raises an error, it is silently ignored by the readline library.
            This behavior makes debugging very difficult. For this reason,
            non-driver methods are run within try-except blocks. When an error
            occurs, the stack trace is printed to self.stderr.

def __driver_stub(self, text, state):
        """Display help messages or invoke the proper completer.

        The interface of helper methods and completer methods are documented in
        the helper() decorator method and the completer() decorator method,
        respectively.

        Arguments:
            text: A string, that is the current completion scope.
            state: An integer.

        Returns:
            A string used to replace the given text, if any.
            None if no completion candidates are found.

        Raises:
            This method is called via the readline callback. If this method
            raises an error, it is silently ignored by the readline library.
            This behavior makes debugging very difficult. For this reason,
            non-driver methods are run within try-except blocks. When an error
            occurs, the stack trace is printed to self.stderr.
        """
        origline = readline.get_line_buffer()
        line = origline.lstrip()
        if line and line[-1] == '?':
            self.__driver_helper(line)
        else:
            toks = shlex.split(line)
            return self.__driver_completer(toks, text, state)