Knowledge-Based Systems

12.6 shallow parsINg 339

restriction for the subject of “kill” is the word “militants” in one sentence,

and the selectional restriction for the object is also the word “militants” in

another sentence. Thus, a semantically symmetric relationship involves sen-

tences where one can swap the subject and object and still end up with a

sentence that makes sense.

Hence, candidate answers fetched on the basis of keyword matching

need to be monitored carefully, as these sentences may have different mean-

ings. In the previous example, the system returns two sentences based on

mere keyword matching. The question in this example is referring to the

sentences, which contain a semantic symmetry relationship.

Two sentences—“National army soldiers ...