Project(Extend(Extend(AggregationJoin(Aggregation(??agg1: ?topic, SAMPLE,
                                                  {},
                                                  G1=Group((?topic),
                                                     BGP( ?s (<http://www.w3.org/2002/01/bookmark#hasTopic>/<http://www.w3.org/2002/01/bookmark#subTopicOf>) ?topic))),
                                      Aggregation(??agg2: ?topic, COUNT,
                                                  {}, G1)),
                      ?topic,
                      ??agg1),
               ?count,
               ??agg2),
        {?topic, ?count})