Joohyung Lee Vladimir Lifschitz Ravi Palla Safe Formulas in the General Theory of Stable Models (Preliminary Report). 672-676 2008 ICLP http://dx.doi.org/10.1007/978-3-540-89982-2_55 conf/iclp/2008 db/conf/iclp/iclp2008.html#LeeLP08