def is_subscribed(subscriber, type_name=None):
    """ 
        Checks to see if 'subscriber' is a subscriber.  'subscriber' can be
        any of the set of supported subscription types (e.g. email address,
        userid).  If type_name is provided, this specifies the intended
        subscription type; if not, the implementation is responsible for
        making a best guess about the intended type.
        """
