Yasunori Ishihara Takuji Morimoto Shougo Shimizu Kenji Hashimoto Toru Fujiwara A Tractable Subclass of DTDs for XPath Satisfiability with Sibling Axes. 68-83 2009 DBPL http://dx.doi.org/10.1007/978-3-642-03793-1_5 conf/dbpl/2009 db/conf/dbpl/dbpl2009.html#IshiharaMSHF09