def unsubscribe(nodeIdentifier, subscriber, requestor):
    """ Cancel the subscription of an entity to a pubsub node.

        The subscription of C{subscriber} is removed from the list of
        subscriptions of the node with id C{nodeIdentifier}. If the
        C{requestor} is not allowed to unsubscribe C{subscriber}, an an
        exception should be raised.

        @return: a deferred that fires when unsubscription is complete.
        """
