Cédric Brun has blogged his investigations into getting Drools to work with Eclipse EMF, so I thought I'd plug his latest entries, with a choice quote and images below:
Live Models Using JBoss Drools and EMF
The 20 minute Graphical Modeler based on Eclipse
"You bet I picked the Rules Engine, so that I can get my hands dirty with those strange beasts you (most of the time) never ever want to meet again after you graduated. I picked JBoss Drools which seemd just nice, powerfull, and based on an implementation of a Rete-like algorithm - which make it fast - and I have to admit I liked their logo is really cool.
EMF and Drools are going along really nicely. Drools considers your Java instances as facts and is then really easy to integrate with EMF. EMF provides the generic notification mechanism which make it really easy to integrate with drools so that drools know that something changed and that he might have some work to do."