doi          = "10.4204/EPTCS.6.1",
 doi          = "10.1016/j.tcs.2006.12.016",
 doi          = "10.1007/978-3-540-31837-8\_14",
 doi          = "10.1007/3-540-29937-8",
 doi          = "10.1007/978-3-540-24619-0\_11",
 doi          = "10.1007/11603047\_20",
 doi          = "10.1007/BF03037282",
 doi          = "10.1007/3-540-10001-6\_30",
 doi          = "10.1007/978-3-540-24619-0\_21",