def invalidate():
    """ Invalidate the session.  The action caused by
        ``invalidate`` is implementation-dependent, but it should have
        the effect of completely dissociating any data stored in the
        session with the current request.  It might set response
        values (such as one which clears a cookie), or it might not.

        An invalidated session may be used after the call to ``invalidate``
        with the effect that a new session is created to store the data. This
        enables workflows requiring an entirely new session, such as in the
        case of changing privilege levels or preventing fixation attacks.
        """
