Consider what would happen in Example 15.44 (page 693) if empty course had been defined as empty course(C) ← course(C) ∧ ∼enrolled(S,C).

Suppose the rest of the knowledge base is course(cs422). course(cs486). course(cs987).

enrolled(huan, cs422). enrolled(sally, cs486).

(a) What is Clark’s completion of the clause for empty course?

(b) What is a counter example to the soundness of the completion? Give an instance of the clause for which the body is true and the head is false.

(c) What does an implementation with negation as failure (e.g., Prolog) give?

Compare with empty course(C) ← ∼enrolled(S,C) ∧ course(C).