Example Horn rule (in made-up syntax): two people with the same name and homepage are the same person:
If { ?x rdf:type foaf:Person. ?y rdf:type foaf:Person. ?x foaf:name ?n. ?x foaf:homepage ?h. ?y foaf:name ?n. ?y foaf:homepage ?h. } then { ?x = ?y }
This work is underway. Not all of it is reflected in latest WD.