4.4.3. Is Logic Programming Mathematical Logic?
Exercise 4.64

An infinite loop

Louis Reasoner mistakenly deletes the outranked-by rule (section 4.4.1) from the data base. When he realizes this, he quickly reinstalls it. Unfortunately, he makes a slight change in the rule, and types it in as

(rule (outranked-by ?staff-person ?boss)
      (or (supervisor ?staff-person ?boss)
          (and (outranked-by ?middle-manager ?boss)
               (supervisor ?staff-person ?middle-manager))))

Just after Louis types this information into the system, DeWitt Aull comes by to find out who outranks Ben Bitdiddle. He issues the query

(outranked-by (Bitdiddle Ben) ?who)

After answering, the system goes into an infinite loop. Explain why.

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