is there something like callback?

[Hidenari]

This question may sound very stupid.
As I'm not familiar with Prolog, this might be a stupid question to
everyone here.

Anyway, I played with Esper before.
It allows you to create a complex event pattern and and you can attach
a callback function.
So that whenever this event happens, you can invoke specific actions
described in the callback.

With Etalis, is it possible to do something like that?

So far, I couldn't find any example that does similar to what I
describe here.
It looks like you have to always query the system to get the events
generated so far.

I am definitely missing something because realtime CEP shouldn't work
like keep querying all the time, right?
If someone show me a good example or some ways to do this kind of
triggering actions, I really really appreciate it.

Thanks,
Hidenari