Mikolaj Bojanczyk Claire David Anca Muscholl Thomas Schwentick Luc Segoufin Two-variable logic on data words. 27 2011 12 ACM Trans. Comput. Log. 4 http://doi.acm.org/10.1145/1970398.1970403 db/journals/tocl/tocl12.html#BojanczykDMSS11