The page itself is under constant construction. There is also a form at the bottom of this page allowing you to submit a faq . The priority of this work will depend on demand, so if you're a potential user who's being shut out by the absence of a Mac version, let us know. We've tested version 3.1.2 of SWI and are very happy with it. SWI Prolog has the further advantage of being free for non-commerical use.
Refraction is a standard concept from the production system literature.
Refracted rules fire just once for any given variable binding. If an element is added to a propositional data store then it appears that all rules which match that particular element will fire only once. This seems to be true even if the element is added, then deleted then added again. Now, when the element is deleted and added again, it will not be exactly the same, allowing the refracted rule to fire again. I can't work out why my rule won't fire.