doi          = {10.2168/LMCS-12(3:13)2016},
 doi          = {10.1007/3-540-60178-3\_85},
 doi          = {10.1215/00294527-2377869},
 doi          = {10.2307/2270284},
 doi          = {10.1002/malq.19940400203},
 doi          = {10.2307/2269016},
 doi          = {10.1016/S0049-237X(08)70849-8},