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we are assuming that R is stratifiable. From the definition of stratification (see
Apt, Blair, and Walker [1988]), it follows that the logic program l\
b
U R is
stratifiable. Przymusinski [1988] shows that a stratifiable logic program has ex-
actly one perfect model. Therefore, each state in S always exists and is unique.
Any two sets of states must contain the same number of states, since there is a
bijection between the set of states and the set of all possible sets of facts. This
proves that if Ν is an M-model, then S always exists and is unique.
The current state G is the perfect model of F U R. Once the database ...