def default_subscriptions():
    """Optionally return a list of default subscriptions.

        Default subscriptions that the module will automatically generate.
        This should only be used in reasonable situations, where users can be
        determined by the event itself.  For instance, ticket author has a
        default subscription that is controlled via trac.ini.  This is because
        we can lookup the ticket author during the event and create a
        subscription for them.  Default subscriptions should be low priority
        so that the user can easily override them.

        :return: a list of tuples (class, distributor, format, priority,
                 adverb)
        """
