doi          = "10.4204/EPTCS.48.8",
 doi          = "10.4204/EPTCS.48.8",
 doi          = "10.1007/978-3-642-02029-2\_2",
 doi          = "10.1017/CBO9781139172752",
 doi          = "10.1016/0168-0072(91)90059-U",
 doi          = "10.1016/j.ic.2009.01.007",
 doi          = "10.1145/1013963.1013978",
 doi          = "10.1016/j.entcs.2002.09.005",
 doi          = "10.1016/0304-3975(93)90091-7",
 doi          = "10.1145/96709.96718",