def subscriptions(requestor, service):
    """
        Called when a subscriptions retrieval request has been received.

        @param requestor: The entity the request originated from.
        @type requestor: L{JID<twisted.words.protocols.jabber.jid.JID>}
        @param service: The entity the request was addressed to.
        @type service: L{JID<twisted.words.protocols.jabber.jid.JID>}
        @return: A deferred that fires with a C{list} of subscriptions as
                 L{Subscription<wokkel.pubsub.Subscription>}.
        @rtype: L{Deferred<twisted.internet.defer.Deferred>}
        """
