Requirements for Rules Engines

IBM DeveloperWorks has an article by Ben Lieberman about the Requirements of Rules Engines. For those of us who are new to Rules Engines (wikipedia) this is a good nuts and bolts description of how they are assembled and what is needed to execute a good program. Ben does a good job outlining the specific of rules engines such as rules definitions, categories of rules, and grouping rules to help formulate decision logic. Even though this article is primarily for the rules engine programmer it is a good comprehensive outline of how the rules engine works.