Stefano Bistarelli Fabio Martinelli Francesco Santini Weighted Datalog and Levels of Trust. 1128-1134 2008 ARES http://doi.ieeecomputersociety.org/10.1109/ARES.2008.197 conf/IEEEares/2008 db/conf/IEEEares/ares2008.html#BistarelliMS08