 More in particular, an extension is defined relative to a default theory. The latter is a pair \((\Gamma, \Delta)\), where \(\Delta\) is a (finite) set of defaults, and \(\Gamma\) is a set of sentences (a world description). The idea is that \(\Gamma\) represents the strict or background information, whereas \(\Delta\) specifies the defeasible information. We say that \(\Xi\) is an extension for a default theory \((\Gamma, \Delta)\) if and only if 