The Lexical Component of START

In order to understand an English sentence, the START system needs to have access to morphological, syntactic, and semantic information about the words in the sentence. All the words that the system is aware of, along with information about their part of speech, inflection, gender, number, etc. are stored in the Lexicon. Virtually every branch of START uses the Lexicon to accomplish its task. In this section we discuss the way in which the Lexicon extends the system's ability to deal with semantic-syntactic interdependencies. We show that the Lexicon provides a place where a verb's membership in a semantic class can be registered, allowing more general S-rules to be stated.

Note that formulating a special purpose S-rule which applies only to the verb surprise does not seem to be the best solution to the problem. Surprise is only one of many verbs which exhibit the so-called property-factoring alternation. This alternation occurs on a large class consisting of over one hundred verbs, among them

(20) amuse, anger, annoy, disappoint, embarrass, frighten, impress, please, scare, stun, ...

These verbs also share a certain semantic property: they all denote emotional reactions. For this reason we identify a class of emotional-reaction verbs and say that the property of the verb surprise responsible for the alternation shown in (10) and (12) holds for all verbs that comprise the emotional-reaction class.

Once we have tied the ability to participate in the property-factoring alternation to a particular class of verbs, we no longer need to indicate this property in the lexical entry of each verb in the class or write verb-specific S-rules, such as the Surprise S-rule. Rather, we can associate the alternation with the emotional-reaction class and then simply indicate in the lexical entry of a verb whether it belongs to this class. That is, we augment a verb's lexical entry with an indication of its semantic class membership. For instance, we would register in the entry for surprise that it is a member of the emotional-reaction class. Now instead of writing a number of verb-specific S-rules, we can write a single general S-rule which triggers on any verb from the emotional-reaction class:

(21) Property-factoring S-rule

      If <<subject verb object1> with object2>

      Then <object2 verb object1>

      Provided verb emotional-reaction class

The revised S-rule contains a Provided clause which specifies the class of verbs to which the rule applies, ensuring that it applies to the emotional-reaction verbs. Provided clauses may impose restrictions on any of the S-rule variables.

When question (14) is asked, the Property-factoring S-rule (used in the backward mode) will trigger, since the T-expression <answer surprise audience> produced by the question matches the Then-part of the rule, and furthermore, the verb surprise belongs to the emotional-reaction class. The correct answer to question (14) is deduced when the appropriately instantiated IF-part of the rule is matched to T-expression (11) found in the knowledge base. Here is how START responds:

      Bill's answer surprised Hillary.

      I deduced that from the following given fact:

      Bill surprised Hillary with his answer.

The Provided restriction of S-rule (21) not only allows the rule to apply to verbs of the appropriate semantic type, but it also prevents the rule from applying to verbs that do not display the property-factoring alternation. For instance, the verbs surprise and present can express their arguments in a similar fashion---both are found in the context [NP V NP with NP], but they differ in the other realizations of their arguments. Specifically present does not participate in the property-factoring alternation, as (22) shows, nor does surprise participate in the alternation that present participates in, as (23) shows:

(22) Hillary presented Bill with a gift.

    *Hillary's gift presented Bill.

(23) Bill surprised the audience with his answer.

    *Bill surprised his answer to the audience.

In the absence of the Provided clause, the Property-factoring S-rule could potentially misapply to verbs like present.

The surprise example shows how the addition of information about semantic class membership to verb entries allows the system to handle a particular phenomenon (or lexical property) common to all verbs in a particular class, with the help of a single S-rule. Note that the verb class approach allows us to extend the system to handle new properties of a class of verbs. All that is required is the addition of the appropriate S-rule, formulated so that it triggers on this class of verbs. There is no need to alter the lexical entries of the members of the class in any way as long as the lexical entry of each verb in the class indicates that it is a member of this class. Thus the verb class approach allows a more modular system design; this in turn allows the coverage of the system to be extended more easily.

By expanding START's knowledge base with additional sentences and augmenting its lexicon with information about synonyms, hyponyms and additional S-rules, we allow the user to ask a larger variety of questions. Suppose that the system was given the following three statements:

    Bill Clinton is the president of the United States of America.

    Hillary Clinton is Bill Clinton's wife.

    Bill Clinton astonished Hillary Clinton with his answer.

Now, in addition to answering questions that closely paraphrase the original statements, START will also be able to answer questions such as:

    Did the answer of the president of the United States of America surprise his wife?

    Was the spouse of the American president stunned by his reply?

    Whose response amazed Hillary?

The examples discussed in this section show how the transparent syntax of S-rules coupled with the information about verb class membership provided by the Lexicon facilitates a more fluent and flexible dialog between the user and the language processing system.