doi          = "10.1007/978-3-540-92687-0",
 doi          = "10.1007/3-540-60117-1-12",
 doi          = "10.1016/0167-6423(95)00025-9",
 doi          = "10.1007/978-3-540-69937-8",
 doi          = "10.1007/978-3-540-92687-0",
 doi          = "10.1007/978-3-540-92687-0",
 doi          = "10.1007/978-3-540-30203-2",
 doi          = "10.1093/logcom/13.5.777",
 doi          = "10.1515/semi.2011.060",
 doi          = "10.4204/EPTCS.81.4",