Allen L. Brown Jr. Surya Mantha Toshiro Wakayama A Logical Reconstruction of Constraint Relaxation Hierarchies in Logic Programming. 362-374 1993 conf/ismis/1993 ISMIS db/conf/ismis/ismis93.html#BrownMW93 http://dx.doi.org/10.1007/3-540-56804-2_34