Complex Event Processing & Rules Engines

I'm currently investigating complex event processors and rules engines.

Windows XP embedded has a rules engine available (through the Windows Workflow Framework). I'm currently trying to figure out what I may have to add to be able to move it into a complex event processor. I think the only thing I need is "state".

But what's state, other than remembered transactions from the past? Therefore, if I simply add a rule that allows some results of the rules engine to persist for a while after processing...

For example, if I need event A and B to happen "close" to each other, I can write something like:

within (1 min)
{
event A condition;
event B condition;
} result (create side effect C)

... is that all I need for a simplistic CEP? If so, why don't more people do it this way?

Still learning...