Xcerpt (cf. here and there) is a declarative, rule-based query and transformation language for the Web, inspired by logic programming.
Xcerpt can be used for querying, possibly in a same query program, both XML and RDF resources.
Relevance
Xcerpt's rule-based queries are an example of rules that might be interchanged on the Web.
Impact on Design
Xcerpt suggests an approach to integrating query languages in a Rule Interchange Format.
For More Information
http://www.mulberrytech.com/Extreme/Proceedings/html/2004/Schaffert01/EML2004Schaffert01.html
http://www.pms.ifi.lmu.de/rewerse-wgi4/software/Xcerpt
http://xcerpt.org
Other
An extension of Xcerpt, called XChange, is a language of Event-Condition-Action rules for reactivity on the Web.