Mikolaj Bojanczyk Claire David Anca Muscholl Thomas Schwentick Luc Segoufin Two-variable logic on data trees and XML reasoning. 10-19 2006 conf/pods/2006 PODS http://doi.acm.org/10.1145/1142351.1142354 db/conf/pods/pods2006.html#BojanczykDMSS06