Introducing S-rules

The T-expressions in the START system are built using the pattern <subject relation object> at every level of embedding and thus mimic the hierarchical organization of English sentences and parallel the representational characteristics of natural language. A language-based knowledge representation system has many advantages: it is very expressive and easy to use; it provides a uniform symbolic representation for parsing and generation; and it makes it possible to automatically create large knowledge bases from natural language texts.

However, a representation mimicking the hierarchical organization of natural language syntax has one undesirable consequence: sentences differing in their surface syntax but close in meaning are not considered similar by the system. Thus, given sentence (3) as input, START will create T-expressions (4), whereas a near paraphrase, sentence (5), will generate T-expressions (6):

(3) Bill surprised Hillary with his answer.

(4) <<Bill surprise Hillary> with answer>

    <answer related-to Bill>

(5) Bill's answer surprised Hillary.

(6) <answer surprise Hillary>

    <answer related-to Bill>

The surprise example illustrates that START needs information that allows it to deduce the relationship between alternate realizations of the arguments of verbs. In this instance, we want START to know that whenever A surprised B with C, then it is also true that A's B surprised C. We do this by introducing rules that make explicit the relationship between alternate realizations of the arguments of verbs. We call such rules S-rules. Here is the S-rule that solves the problem caused by the verb surprise:

(7) Surprise S-rule

    If <<subject surprise object1> with object2>

    Then <object2 surprise object1>

S-rules are implemented as a rule-based system. Each S-rule is made up of two parts, an antecedent (the If-clause) and a consequent (the Then-clause). Each clause consists of a set of templates for T-expressions, where the template elements are filled by variables or constants. The Surprise S-rule will apply only to T-expressions which involve the verb surprise and which meet the additional structural constraints.