Robert E. Mercer Vincent Risch Properties of maximal cliques of a pair-wise compatibility graph for three nonmonotonic reasoning system. 2003 conf/asp/2003 Answer Set Programming http://SunSITE.Informatik.RWTH-Aachen.de/Publications/CEUR-WS//Vol-78/asp03-final-mercer-clic.pdf db/conf/asp/asp2003.html#MercerR03