Ravi Kumar Kunal Punera Torsten Suel Sergei Vassilvitskii Top-<i>k</i> aggregation using intersections of ranked inputs. 222-231 2009 WSDM http://doi.acm.org/10.1145/1498759.1498830 conf/wsdm/2009 db/conf/wsdm/wsdm2009.html#KumarPSV09