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).

Business Rules: BRE Patterns : Collaboration Cop, Part I