Prolog is rather popular for Expert Systems. Why? Well because it's homoiconic and because we have DCG's, we can reify a query to make it explain itself. That's a lot of jargon, in this post we'll break it down and make some explanations.

Under the open world assumption a fact can be True, False or Unknown. Prolog operates in a closed world where predicates are either True or False. Let's see if we can open Prolog up a little.