Distinct(Project(Filter((?c1 != ?c2),
                        BGP( ?t1 <http://www.w3.org/2000/01/rdf-schema#comment> ?c1 .
                             ?t1 <http://www.w3.org/2000/01/rdf-schema#comment> ?c2)),
                 {?t1, ?c1, ?c2}))