doi          = "10.1007/978-3-642-02273-9\_4",
 doi          = "10.1016/j.ic.2008.10.003",
 doi          = "10.1145/2159531.2159533",
 doi          = "10.4230/LIPIcs.CSL.2012.77",
 doi          = "10.2307/2275524",