doi          = "10.1007/3-540-44802-0\_28",
 doi          = "10.1006/inco.1996.0025",
 doi          = "10.2307/2694930",
 doi          = " 10.1016/S0020-0190(02)00300-9",
 doi          = "10.1007/BFb0013061",
 doi          = "10.1109/LICS.2005.48",
 doi          = "10.1007/978-3-642-12251-4\_11",
 doi          = "10.1145/1805950.1805958",
 doi          = "10.1007/BFb0073967",
 doi          = "10.1137/0211062",
 doi          = "10.1017/S0956796898003141",
 doi          = "10.1007/978-3-642-21691-6\_18",