Rule-Based Programming and Magic

Although I don't have time to work on Laterna Magica right now, I have the occasional thought on it. Before the big pause, I noticed a major flaw in my implementation of characteristics. The problem is that every read-operation on the characteristics refreshes the status. This status change means that only in-game operations may read characteristics; I mentioned a couple of times that, to achieve independence of game and GUI, the GUI must not change the game state.

...and what does this mean in the context of rules-based programming? Let me first explain that term in greater detail: In the last months, we familiarized with the (unfortunately commercial) rule-based programming language "JESS", its interpreter is implemented in Java.
A rule based system operates on a fact base. Facts can be roughly compared to variables or objects. The second important part is, of course, the rules. Rules operate on facts and may be activated whenever a fact changes. An activated rule is not immediately executed! Instead, a call to run() causes the engine to process the activated rules, which may in turn change facts and activate even more rules.

And now combine the two thoughts: The reason for me to re-evaluate characteristics every time they are accessed was to make sure no out-of-date data is used. With a rule-based system keeping track of the game state, this becomes unnecessary, because the rule system would ensure that characteristics are refreshed when necessary. And without the need to do permanent refreshes, there's no problem with the GUI reading characterics.

If anyone knows some free, Java-based rules engines, this would be a great input. I haven't done any research on this, and I won't do it until I come around programming LM again, but concrete user stories and recommendations are always great to hear.