These can be used to layer a degree of sequential control over the set-based pattern matching approach taken by engines like MS BRE. The basic pattern is simplicity itself, and very common. What I generally do is assert an additional 'context' fact to the engine (typically some custom .NET object) in which I maintain a state specifier (e.g., a simple string property).

The basic idea is that you create an object model that corresponds to your "facts," write rules to produce new objects or "facts," then harvest the newly created facts from the engine or wherever you've stored them.

Business Rules: BRE Patterns : Collaboration Cop, Part I