Checking if a program has an answer set, and if so, compute its answer sets are just some of the important problems In answer set logic progremming. Solving these problems using Gelfond and Llfschltz＇s original definition of answer sets Is not an easy task. Alternative charaoterlzatlons of answer sets for nested logic programs by Erdem and Llfschltz, Lee and Llfschltz, and You at el. are based on the completion semantics and various notions of tlghtnese. However, the notion of tightness Is a local notion In the sense that for different answer sets there are, In general, different level mappings capturing their tlghtnese. This makes It hard to be used In the deelgn of algorithms for computing answer sets. This paper proposes a charecterization of answer sets based on sets of generetlng rules. From this charaoterlzation new algorithms are derived for computing answer sets and for performing some other reasoning teaks. As an application of the charecterlzatlon a sufficient and necessary condition for the equivalence between answer set sementics and completion semantics has been proven, and a basic theorem Is shown on computing answer sets for nested logic programs baaed on an extended notion of loop formulas. These results on tlghtnese and loop formulas are more general than that in You and Lin＇a work.
Based on logic programs, authorization conflicts and resolution strategies are analyzed through the explanation of some examples on the health care sector. A resolution scheme for handling conflicts in high level authorization specification by using logic program with ordered disjunction （LPOD） is proposed. The scheme is useful for solving conflicts resulted from combining positive and negative authorization, complexity of authorization management, and less clarity of the specification. It can well specify kinds of conflicts （such as exceptional conflicts, potential conflicts）, and is based on literals and dependent contexts. Thus it is expressive and available. It is shown that authorizations based on rules LPOD is very important both in theory and practice.
