def __init__(self, pattern, response_dict):
        """
        Configure XQueue stub to POST `response_dict` (a dictionary)
        back to the LMS when it receives a submission that contains the string
        `pattern`.

        Remember that there is one XQueue stub shared by all the tests;
        if possible, you should have tests use unique queue names
        to avoid conflict between tests running in parallel.
        """
        self._pattern = pattern
        self._response_dict = response_dict