Wim Martens Frank Neven Frontiers of Tractability for Typechecking Simple XML Transformations. 2004 conf/pods/2004 PODS http://doi.acm.org/10.1145/1055558.1055563 http://www.acm.org/sigmod/pods/proc04/pdf/P-03.pdf db/conf/pods/pods2004.html#MartensN04 23-34