(7th-August-2020)
• The second type of error occurs when an expected answer is not produced. This manifests itself by a failure when an answer is expected. A goal g that is true in the domain, but is not a consequence of the knowledge base, is called a false-negative error.
• The preceding algorithm does not work in this case. There is no proof. We must look for why there is no proof for g.
• An appropriate answer is not produced only if a definite clause or clauses are missing from the knowledge base. By knowing the intended interpretation of the symbols and by knowing what queries should succeed (i.e, what is true in the intended interpretation), a domain expert can debug a missing answer. Given an atom that failed when it should have succeeded, Figure 5.7 shows how to find an atom for which there is a missing definite clause.
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