doi          = "10.1016/S1571-0661(04)80487-0",
 doi          = "10.2307/2273659",
 doi          = "10.1017/CBO9781139032636",
 doi          = "10.4230/LIPIcs.CSL.2012.243",
 doi          = "10.1007/978-3-642-21691-6_15",
 doi          = "10.1007/978-3-642-29485-3_16",