 Solution

ID: past-exam-of-the-computer-science-course-of-the-university-of-oxford/2015/knowledge-representation-and-reasoning/3/c/ii/solution

The remaining classification rule is

Year (x) \land not Leap (x) \to Common (x) .

The positive premise restricts the rule to years. Its negation as failure premise is evaluated by stable model semantics; it does not assert a separately stored negative fact. This gives a stratified Datalog program: divisibility and year facts are inputs, leap status is computed next, and common status last.

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