'Executed after a Rule matches.
Should yield CallBackFuture instances which will then be passed into
the given callback.
See the notification implementation for example usage.
>>> def after(self, event, state):
>>>     yield self.future(self.print_results)
>>> def print_results(self, event, futures):
>>>     print(\'Got futures for Event {}\'.format(event.id))
>>>     for future in futures:
>>>         print(future)'

def after(self, event, state):

    raise NotImplementedError
