Maurice Bruynooghe Henk Vandecasteele D. Andre de Waal Marc Denecker Detecting Unsolvable Queries for Definite Logic Programs. 118-133 http://link.springer.de/link/service/series/0558/bibs/1490/14900118.htm 1998 conf/plilp/1998 PLILP/ALP db/conf/plilp/plilp98.html#BruynoogheVWD98