1. found in Eiter et al. (2009), Ferraris and Lifschitz (2005), Gelfond (2008), Lifschitz (1996, 2004).
2. is explored in depth in the area of Nonmonotonic Reasoning (Besnard, 1989, Ginsberg, 1987,
3. tonic Reasoning, inparticular, Autoepistemic (Moore, 1985) andDefault Logic (Reiter, 1980).