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<?xml version="1.0" standalone="yes"?> <Paper uid="J86-1001"> <Title>RESOLVING LEXICAL AMBIGUITY IN A DETERMINISTIC PARSER</Title> <Section position="4" start_page="0" end_page="0" type="metho"> <SectionTitle> 2 SYNTACTIC CONTEXT 2.1 WORD DATA STRUCTURES </SectionTitle> <Paragraph position="0"> As a first approach to handling ambiguity, it was asked, &quot;If we construct a compound lexical entry for each word composed of the features of each part of speech the word can have and make no alterations to the grammar, how wide a coverage of examples will we get?&quot; This approach was used by Winograd (1972) and was found to be very effective for the following reason. Each word has all the possible relevant features for it. Therefore, the test will succeed for each possible part of speech with which a word can be used. In this way, all applicable rules will match. It may be that often only one rule will match, or that the first rule tried is the correct rule.</Paragraph> <Paragraph position="1"> The question is, how often will the rule that matches be the correct rule? All words in ROBIE are defined in the syntactic dictionaries. Each word has a compound lexical entry incorporating all the features for all the possible parts of speech the word could have. This is exactly as was done by Winograd (1972). For example, block is defined as a noun and a verb, can is defined as a noun, auxiliary verb, and verb, and hit is defined as a noun and a verb. The features for each of these parts of speech are kept in the dictionary and, when the word is looked up, they are returned as a single ordered list of features. These features are sub-grouped according to the part of speech they are associated with. Hence, when the word block is looked up, the result returned is both the noun and the verb definition. In this way, all possibilities are returned. In the English language, most words can have several parts of speech. This fact must be reflected in a parser of English and we do this with the multiple meanings above.</Paragraph> <Paragraph position="2"> When the parser has enough information to decide which is the correct part of speech, it ignores (removes) the other possibilities. In this way, we have not built structure that is later thrown away. Although some may argue that this is a form of parallelism, it seems necessary since it reflects the inherent parallelism of language.</Paragraph> <Section position="1" start_page="0" end_page="0" type="sub_section"> <SectionTitle> 2.2 MORPHOLOGY </SectionTitle> <Paragraph position="0"> The first part of the disambiguation process takes place in the morphology. When ROBIE identifies a word that has a morphological ending, the morphology must adjust the features of the word. For example, when blocked is identified, the feature &quot;ed&quot; must be added to the list of features for block. At the same time, a portion of the disambiguation takes place. If block is defined as both a noun and a verb, then blocked is not a noun. The morphology causes some features to be added, such as &quot;ed, past&quot; and some features to be removed such as &quot;tenseless.&quot; As features that are no longer applicable are removed, so also are parts of speech and their associated features that are no longer applicable. For blocked, the features &quot;noun, ns, n3p&quot; will be removed and the features &quot;adjective, ed, past&quot; will be added.</Paragraph> <Paragraph position="1"> The morphology will identify words such as adverbs, adjectives, and verbs in a similar way. The morphology used is very similar to that of Winograd (1972) and of Dewar, Bratley, and Thorne (1969); the part-of-speech additions and deletions are taken from Marcus (1980).</Paragraph> <Paragraph position="2"> Although this technique may seem obvious, it is included to point out that a majority of the occurrences of part-of-speech ambiguity can be resolved or reduced on the basis of the morphology alone.</Paragraph> </Section> <Section position="2" start_page="0" end_page="0" type="sub_section"> <SectionTitle> 2.3 DISAMBIGUATION </SectionTitle> <Paragraph position="0"> Now that we have allowed words to have multiple parts of speech and the morphology can be used to trim some of the ambiguity, we need a simple technique for disambiguating words to a single part of speech. Again, referring to Occam's Razor, what is preferable is a simple and general technique for all types of disambiguation.</Paragraph> </Section> </Section> <Section position="5" start_page="0" end_page="0" type="metho"> <SectionTitle> 2 Computational Linguistics, Volume 12, Number 1, January-March 1986 Robert Milne Resolving Lexical Ambiguity in a Deterministic Parser </SectionTitle> <Paragraph position="0"> In ROBIE each rule matches the features of one or two buffer cells. (The word buffer will be used interchangeably with cell. That is, buffer and cell are the same concept.) If the word block is in the first buffer cell, then a pattern \[noun\] or a pattern \[verb\] will match. These patterns do not relate to the other possible definitions of a word. If a rule pattern has matched on the feature &quot;noun&quot; in the first buffer cell, then ROBIE assumes that this word is a noun. It would then be appropriate to disambiguate the word as a noun. This is exactly as in Winograd (1972).</Paragraph> <Paragraph position="1"> In a non-deterministic parser, it is not essential to find the correct rule first. If the parser runs an incorrect rule, the parser may backtrack and change the category assignment. But in a deterministic parser, there will never be any backtracking, and this solution cannot be used.</Paragraph> <Paragraph position="2"> Since ROBIE does not backtrack, disambiguating the word when the pattern matches will always result in the same disambiguation as if the word were disambiguated in the grammar rule. Once a rule runs assuming a buffer contains a certain part of speech, it must be used as such in the parser. The general disambiguation scheme is: if a full pattern matches a word as a certain part of speech, then it is disambiguated as that part of speech.</Paragraph> <Paragraph position="3"> The compound lexical entries and pattern-matching disambiguation alone will handle many examples of ambiguity. In the rest of this paper we see just what this can do for us.</Paragraph> <Section position="1" start_page="0" end_page="0" type="sub_section"> <SectionTitle> 2.4 AN EXAMPLE </SectionTitle> <Paragraph position="0"> Given the above mechanisms - multiple definition and disambiguation by the pattern matching, let us see how a few simple examples are handled. Consider: (1) The falling block needs painting.</Paragraph> <Paragraph position="1"> We will look only at the words falling and block in this example. The word falling is defined as a verb and an adjective in the dictionary and block is defined as a noun and a verb.</Paragraph> <Paragraph position="2"> While parsing this example, after the word the has initiated an NP and been attached to it as a determiner, the rules to parse adjectives are activated. The rule ADJECTIVE has the pattern \[adj\], and matches the word falling. Falling is then attached and disambiguated as an adjective. Recognition of falling as a verb does not occur. As there are no more adjectives, ROBIE will activate the rules to parse the headnoun. (ROBIE's grammar assumes that all words between the first noun and the head noun of an NP are nouns; see section 2.6.) The rule NOUN with the pattern \[noun\] will match on the word block, and it will be attached as a noun. Hence block will also be disambiguated without the verb use being considered by ROBIE.</Paragraph> <Paragraph position="3"> Other ambiguities inside the noun phrase will be handled in a similar way. This approach will usually cover the situation of singular head nouns, verb/adjective ambiguity and many other pre-nominal ambiguities. This works because the noun phrase has a very strict word order. When an ambiguous word is found, only one of its meanings will be appropriate to the word order of the noun phrase at that point. This approach can be thought of as an extension of the basic approach of the Harvard Predictive Analyzer (Kuno 1965).</Paragraph> <Paragraph position="4"> This strategy will also often disambiguate main verbs. For example, consider the following sentences: (2) Tom hit Mary.</Paragraph> <Paragraph position="5"> (3) Tom will hit Mary.</Paragraph> <Paragraph position="6"> (4) The will gave the money to Mary.</Paragraph> <Paragraph position="7"> In (2), hit is the main verb. In the dictionary, hit is also defined as a noun, (as in card playing). The parser will attach Tom as the subject of the sentence and then activate the rules for the main verb. Since hit has the feature &quot;verb&quot;, it will match that rule and be attached and disambiguated as a verb. Again, other possible parts of speech are not considered.</Paragraph> <Paragraph position="8"> The word will could be a noun or a modal as sentences (3) and (4) demonstrate. In (3), will cannot be part of the headnoun with Tom, so the NP will be finished as above. The rules for the auxiliary will then be activated and the word will then matches the pattern \[modal\] and is attached to the AUX.</Paragraph> <Paragraph position="9"> In (4), the word will is used as a noun. Since it follows the determiner, the rules for nouns will be activated. The word will then matches the pattern \[noun\] and attaches to the NP as a noun.</Paragraph> <Paragraph position="10"> The same approach will also disambiguate stop and run in the following sentence. Since stop is sentence initial and can be a tenseless verb, the rule IMPERATIVE will match, and it will be disambiguated as a verb. The word run, which can be a noun or a verb, will be handled as will in (4).</Paragraph> <Paragraph position="11"> (5) Stop the run.</Paragraph> </Section> </Section> <Section position="6" start_page="0" end_page="0" type="metho"> <SectionTitle> 2.5 THE WORD TO </SectionTitle> <Paragraph position="0"> NOW let us consider a more difficult example, the word to. To is defined as an auxiliary verb and a preposition in ROBIE, as illustrated by these sentences: (6) I want to kiss you.</Paragraph> <Paragraph position="1"> (7) I will go to the show with you.</Paragraph> <Paragraph position="2"> In (6), to is the infinitive auxiliary, while in (7) to is a preposition. This analysis is based on that of Marcus (1980:118). Our two buffer cell lookahead is sufficient to disambiguate these examples.</Paragraph> <Paragraph position="3"> The buffer patterns for the above sentences are:</Paragraph> <Paragraph position="5"> By looking at the following word, to can be disambiguated. In (7), the word the cannot be a tenseless verb, so the first pattern does not match. In (6), the second buff-Computational Linguistics, Volume 12, Number 1, January-March 1986 3 Robert Milne Resolving Lexical Ambiguity in a Deterministic Parser er does not have the feature &quot;ngstart&quot;, so the rule doesn't match.</Paragraph> <Paragraph position="6"> However, the above patterns will accept ungrammatical sentences. To reject ungrammatical sentences, we can use verb subcategorisation as a supplement to the above rules. One cannot say: (8) *I want to the school with you.</Paragraph> <Paragraph position="7"> (9) *I will hit to wash you.</Paragraph> <Paragraph position="8"> In English, only certain verbs can take infinitive complements. To can only be used as an auxiliary verb starting a VP when the verb can take an infinitive complement. Hence, by activating the rules to handle the VP usage only when the infinitive is allowed, the problem is partly reduced. Also by classifying the verb for PPs with the preposition to, the problem is simplified. This is merely taking advantage of subcategorisation in verb phrases. Taking advantage of this subcategorisation greatly reduces, but does not eliminate, the possible conflict.</Paragraph> <Paragraph position="9"> We have seen what to do if the verb will only accept a toPP or a VP. The final difficult situation arises whenever the following three conditions are true: * the verb will accept a toPP and a toVP, * the item in the second buffer has the features &quot;tenseless&quot; and &quot;ngstart&quot; and, * the toPP is a required modifier of the verb.</Paragraph> <Paragraph position="10"> Although this situation rarely arises, the above rule will make the wrong decision if the ambiguous word is being used as a noun. In this situation, ROBIE will make the wrong decision, and has no capability to better decide. By default, the principles of Right Association and Minimal Attachment apply as discussed in Frazier and Fodor (1978).</Paragraph> <Paragraph position="11"> A free text analysis done on a cover story in TIME magazine (1978) resulted in 55 occurrences of the word to. The two rules mentioned above in conjunction with verb subcategorisation gave the correct interpretation of all of these. These rules were also checked on the MECHO corpus (Milne 1983) and the ASHOK corpus (Martin, Church, and Patil 1981). There were no violations of these rules in either of these.</Paragraph> <Section position="1" start_page="0" end_page="0" type="sub_section"> <SectionTitle> 2.6 ADJECTIVE/NOUN AND NOUN/NOUN AMBIGUITY </SectionTitle> <Paragraph position="0"> Adjective/noun ambiguity is beyond the present scope of this research and is handled in a simple-minded way. If the word following the ambiguous adjective/noun word can be a noun, then the ambiguous word is used as an adjective. In other words, all conflicts are resolved in favour of the adjective usage. This problem arises in these examples: (10) The plane is inclined at an angle of 30 degrees above the horizontal.</Paragraph> <Paragraph position="1"> (11) A block rests on a smooth horizontal table. In (10), horizontal is a noun, while in (11), it is an adjective. The above algorithm handles these cases. This approach takes advantage of the lookahead of the deterministic parser. A word should be used as an adjective if the following word can be an adjective or a noun. However, this approach would fail on examples such as: (12) The old can get in for half price.</Paragraph> <Paragraph position="2"> (13) The large student residence blocks my view. 2.7 WHY DO THESE TECHNIQUES WORK? In this section we have seen many examples of the resolution of ambiguity. To handle these examples, we merely constructed a compound lexical entry for each word, composed of the features of each part of speech the word could be and allowed the pattern matching to perform the disambiguation. This technique has been used by Winograd (1972). Why does this work so well? English has a fairly strict structural order for all the examples presented here. Because of this, in each example we have seen, the use of the word as a different part of speech would be ungrammatical. Although these techniques have been used for non-deterministic parsers, their effectiveness has not been investigated for a deterministic parser.</Paragraph> <Paragraph position="3"> Most ambiguities are not recognised by people because only one of the alternatives is grammatical. In many situations, when fixed constituent structure is taken into account, other uses of an ambiguous word are not possible and probably not even recognised. Since fixed constituent structure rules out most alternatives, we have been able to handle the examples in this paper without any special mechanisms. In the introduction to this paper, it was stated that a clean and simple method of handling ambiguity was desired. I feel that this goal has been met for these examples.</Paragraph> </Section> </Section> <Section position="7" start_page="0" end_page="0" type="metho"> <SectionTitle> 3 THE ROLE OF AGREEMENT IN HANDLING AMBIGUITY </SectionTitle> <Paragraph position="0"> Using the simple techniques presented in the last sections, we can handle many cases of part-of-speech ambiguity, but there are many examples we cannot resolve. For example, the second of each pair of sentences below would be disambiguated incorrectly.</Paragraph> <Paragraph position="1"> (14) I know that boy is bad.</Paragraph> <Paragraph position="2"> (15) I know that boys are bad.</Paragraph> <Paragraph position="3"> (16) What boy did it? (17) What boys do is not my business.</Paragraph> <Paragraph position="4"> (18) The trash can be smelly.</Paragraph> <Paragraph position="5"> (19) The trash can was smelly.</Paragraph> <Paragraph position="6"> Many people wonder what role person/number codes and the relatively rigid constituent structure in the verb group play in English. In this section, we will explore their role by attempting to answer the question, &quot;What use is the fixed structure of the verb group and person/number codes?&quot;</Paragraph> </Section> <Section position="8" start_page="0" end_page="0" type="metho"> <SectionTitle> 4 Computational Linguistics, Volume 12, Number 1, January-March 1986 </SectionTitle> <Paragraph position="0"/> <Section position="1" start_page="0" end_page="0" type="sub_section"> <SectionTitle> Robert Milne Resolving Lexical Ambiguity in a Deterministic Parser 3.1 UNGRAMMATICAL SENTENCES </SectionTitle> <Paragraph position="0"> Before we proceed, let us look at an assumption Marcus made in his parser, that it would be given only grammatical sentences. This assumption makes life easy for someone writing a grammar, since there is no need to worry about grammatical checking. Hence no provision was made for ungrammatical sentences and the original parser accepted such examples as: (20) *A blocks are red.</Paragraph> <Paragraph position="1"> (21) *The boy hit the girl the boy the girl.</Paragraph> <Paragraph position="2"> (22) *Are the boy run? This simplification causes no problems in most sentences, but can lead to trouble in more difficult examples. If the parser's grammar is loosely formulated because it assumes it will be given grammatical examples only, then ungrammatical sentences may be accepted. If the syntactic analysis accepts ungrammatical sentences as grammatical, then it is making an error. Using grammatical constraints actually helps parsing efficiency and disambiguation. In the next sections we look at the consequences of this assumption as well as those of rejecting ungrammatical sentences.</Paragraph> </Section> <Section position="2" start_page="0" end_page="0" type="sub_section"> <SectionTitle> 3.2 SUBJECT/VERB AGREEMENT </SectionTitle> <Paragraph position="0"> We know that the verb group has a complicated but relatively fixed constituent structure. Although verbals have many forms, they must be mixed in a certain rigid order.</Paragraph> <Paragraph position="1"> We also know that the first finite verbal element must agree with the subject in person and number. That is, one cannot say: (23) *The boy are run.</Paragraph> <Paragraph position="2"> (24) *The boy will had been run.</Paragraph> <Paragraph position="3"> (25) *The boys had are red.</Paragraph> <Paragraph position="4"> etc.</Paragraph> <Paragraph position="5"> While Marcus's parser enforced these observations to some extent, he did not follow them throughout his parser. We want to enforce this agreement throughout ROBIE. Checking the finite or main verb, to be sure that it agrees in number with the subject, will lead to the rejection of the above examples. This was done by adding the agreement requirement into the pattern for each relevant rule as will be explained later. Buffers 1 and 2 must agree before a rule relating the subject and verb can match. This check looks at the number code of the NP and the person/number code of the verb and checks whether they agree. The routine for subject/verb agreement is very general and is used by all the subject/verb rules. The routine can only check the grammatical features of the buffers.</Paragraph> </Section> </Section> <Section position="9" start_page="0" end_page="0" type="metho"> <SectionTitle> 3.3 MARCUS'S DIAGNOSTICS </SectionTitle> <Paragraph position="0"> Marcus (1980) did handle some part-of-speech ambiguities. The words to, what, which, that, and have could all be used as several parts of speech. For each of these words he also used a Diagnostic rule. These Diagnostic rules matched when the word they were to diagnose arrived in the first buffer position and the appropriate packets were active. Each diagnostic would examine the features of the three buffers cells and the contents of the Active Node Stack. Once the diagnostic decided which part of speech the word was being used as, it either added the appropriate features, or explicitly ran a grammar rule. Marcus did not give each word a compound lexical entry as we have done here.</Paragraph> <Paragraph position="1"> Most of the grammar rules in his parser were simple and elegant, but the diagnostics tended to be very complex and contained many conditionals. In some cases they also seemed rather ad hoc and did not meet the goal of a simple, elegant method of handling ambiguity.</Paragraph> <Paragraph position="2"> For example, consider the THAT-DIAGNOSTIC: \[that\]\[np\] -* in the Packet CPOOL (Clause pool of rules) &quot;If there is no determiner of second and there is not a qp of second and the nbar of 2nd is none of massn,npl and 2nd is not-modifiable then attach as det else if c is nbar then label 1 st pronoun, relative pronoun else label 1st complementiser.&quot; (Marcus 1980:291) Notice that if the word that were to be used as a determiner, then it would be attached after the NP was built! This is his primary rule for disambiguating the word that. Marcus's parser also had three other rules to handle different cases.</Paragraph> <Paragraph position="3"> It seems that these rules did not &quot;elegantly capture generalisations&quot; as did the rest of his parser. I consider these rules undesirable and feel that they should be corrected to comply with my criteria for simple and elegant techniques in resolving ambiguity. I wanted a method that used no special mechanism, or routine, other than that needed to parse grammatical sentences. These diagnostics are certainly special mechanisms and do not meet this goal. Can we cover the same examples in a more simple and principled way? In this section, we look at each of these diagnostics in turn and show how they have been replaced in the newer model. We also look at a few other examples of ambiguity which Marcus did not handle, but are related to our discussion here.</Paragraph> </Section> <Section position="10" start_page="0" end_page="0" type="metho"> <SectionTitle> 3.4 HANDLING THE WORD TO </SectionTitle> <Paragraph position="0"> The handling of to by Marcus's diagnostic can be replaced by the method outlined in Section 2.5. This method was motivated to handle grammatical sentences and meets our criterion for a simple approach.</Paragraph> </Section> <Section position="11" start_page="0" end_page="0" type="metho"> <SectionTitle> 3.5 HANDLING WHATAND WHICH </SectionTitle> <Paragraph position="0"> For both what and which, the ambiguity lies between a relative pronoun and a determiner. The following examples show various uses of both words: Computational Linguistics, Volume 12, Number 1, January-March 1986 5 Robert Milne Resolving Lexical Ambiguity in a Deterministic Parser (26) Which boy wants a fish? det (27) Which boys want fish? det (28) The river which I saw has many fish. tel. pron.</Paragraph> <Paragraph position="1"> (29) What boy wants a fish? det (30) What boys want is fish. tel. pron.</Paragraph> <Paragraph position="2"> There is some debate about the part of speech to be assigned the word which. Some linguists consider it to be a quantifier (Chomsky 1965), while others consider it to be a determiner (Akmajian and Heny 1975, Chapter 8). We shall adopt the determiner analysis, making the problems for what and which similar.</Paragraph> <Paragraph position="3"> To determine the correct part of speech for these two words, Marcus (1980:286) used the following diagnostics: null \[which\] -~ in the packet CPOOL &quot;If the NP above Current Node is not modified then label 1st pronoun, relative pronoun else label 1st quant,ngstart,ns,wh,npl.&quot; \[what\]\[t\] -*- in the packet NPOOL &quot;If 2nd is ngstart and 2nd is not det then label 1st det,ns,npl,n3p,wh; activate parse det else label 1st pronoun,relpron,wh.&quot; These diagnostics would make the word in question a relative pronoun if it occurred after a headnoun, or a determiner if the word occurred at the start of a possible noun phrase.</Paragraph> <Paragraph position="4"> If we follow the approach in the last section, and give each word a compound lexical entry composed of the determiner and relative pronoun features, we find that these words are always made determiners unless they occur immediately after a headnoun. In other words, the which examples are all parsed correctly, but (30) is parsed incorrectly. This happens because the determiner rule will always try to match before the rule for WH questions can take effect. This simple step gives the correct analysis if the ambiguous word is to be a determiner, but will still err on (30).</Paragraph> <Paragraph position="5"> The rule to parse a relative pronoun and start a relative clause is active only after the headnoun has been found. At this time, the rule for determiners is not active. Therefore, if the word what or which is present after a headnoun, the only rule that can match is the rule to use it as a relative pronoun, and it will be used as a relative pronoun. We have resolved the simple case of what as a relative pronoun using only the simple techniques of the last section. For these sentences (31) What block is red? (32) Which boy hit her? (33) Which is the right one? ROBIE produces the correct analysis, but still errs on (30). This error is because what is being used as a relative pronoun but does not follow a headnoun. Without any additional changes to the parser, we get two things. Firstly, if the word occurs after the headnoun, then the NP-COMPLETE packet rules are active, and it will be a relative pronoun. In fact, since relative clauses can occur only after the end of an NP, this correctly resolves the relative pronoun uses. If the word occurs at the start of an NP, then it will be made a determiner.</Paragraph> <Paragraph position="6"> This approach has exactly the same effect and coverage as did Marcus's diagnostics, but we have not needed any special rules to implement it. It will now provide the correct interpretation for which, but will make some errors for the word what. Marcus's what-diagnostic will treat what as a determiner whenever the item in the second buffer could start a NP. This is usually correct, but what will be treated as a determiner in all of the following: (34) What boys want is fish.</Paragraph> <Paragraph position="7"> (35) What blocks the road? (36) What climbs trees? (37) What boys did you see? (38) What blocks are in the road? (39) What climbs did you do? In this paper, we are adopting the following analysis for WH clefts such as (34). The initial WH word, what is a relative pronoun and attached as the WH-COMP of the subject S node. The subject is the phrase What boys want. The main verb of the sentence is is and the complement fish. The exact details are not important, only that the word what or which is a not determiner at the start of a WH cleft.</Paragraph> <Paragraph position="8"> In sentences (34-36), the word what is not used as a determiner. In the analysis we are using, it is a relative pronoun and is used as the WH-COMP for the S. In sentences (37-39), the word what is used as a determiner. Marcus (1980:286) admits that this diagnostic produces the incorrect result in this case. His diagnostic will make what a determiner in all of these examples, as will my analysis.</Paragraph> <Paragraph position="9"> One can also see that each of the above pairs is a pair of potential garden path sentences. For each pair, the two buffers contain the same words. Hence our two-buffer lookahead is not sufficient to choose the correct usage of the word what. Using only two or three buffers, there is no way to make what a relative pronoun when the headnoun is plural but a determiner when it is singular for all arbitrary sentences.</Paragraph> <Paragraph position="10"> With regard to the Semantic Checking Hypothesis (Milne 1982) then, it is suggested that this decision is based on non-syntactic information. I believe that intonation is critical in these examples. Unfortunately there is insufficient experimental evidence to determine for certain whether this is true. Finally, the problem of what and which as sentence initials, with no noun in the second buffer seems to arise very rarely. I have found no examples of this problem in free text analysis.</Paragraph> <Paragraph position="11"> The current parser (ROBIE) cannot obtain the extra information provided by intonation to help resolve this case. As a result it follows Mareus's diagnostic and makes what a determiner in each of the above cases.</Paragraph> </Section> <Section position="12" start_page="0" end_page="0" type="metho"> <SectionTitle> 6 Computational Linguistics, Volume 12, Number 1, January-March 1986 Robert Milne Resolving Lexieal Ambiguity in a Deterministic Parser </SectionTitle> <Paragraph position="0"> This is because what is defined as a determiner that can agree with either a singular noun or a plural noun, as it was in Marcus's parser.</Paragraph> <Section position="1" start_page="0" end_page="0" type="sub_section"> <SectionTitle> 3.6 HANDLING THAT </SectionTitle> <Paragraph position="0"> In ROBIE, that is defined as a singular determiner, a pronoun, a relative pronoun, and a complementiser.</Paragraph> <Paragraph position="1"> Marcus had four diagnostics to handle the word that. We have seen one of these at the start of this section. In this sub-section we see how these four diagnostics can be replaced in a simple way. Let us consider how to handle the uses of that one at a time.</Paragraph> <Paragraph position="2"> Firstly, as a determiner. The following sentences illustrate the problem in identifying this usage.</Paragraph> <Paragraph position="3"> (40) I know that boy should do it.</Paragraph> <Paragraph position="4"> (41) I know that boys should do it.</Paragraph> <Paragraph position="5"> Marcus assumed that PARSIFAL would be given only grammatical sentences to parse. If determiner/number agreement is not given to a parser, then it will, incorrectly, make that a determiner in (41), producing the wrong analysis. The way to prevent this is to enforce number agreement in the rule DETERMINER by insisting that the determiner agree with the noun in number. The determiner usage will be grammatical only when the headnoun has the same number. If we make this a condition for the rule to match, then that will not be made a determiner in (41) and ROBIE will get the correct parse.</Paragraph> <Paragraph position="6"> For this case, the agreement check would make sure that one of the following patterns match: \[det,ns\] \[noun,ns\] \[det,npl\] \[noun,npl\] The above two cases are handled properly because number agreement blocks the interpretation of the (41) as a determiner. This approach leads to the correct preference, when there is an ambiguity and accounts for the difficulty in (42) versus (43): (42) That deer ate everything in my garden surprised me.</Paragraph> <Paragraph position="7"> (43) That deer ate everything in my garden last night. The second experiment in Milne (1983), showed that (42) is a garden path sentence, while (43) is not. In both sentences, it is believed the subject uses the word that as a determiner. Deer is both singular and plural, so it fits the above rule. In (42), that must be used as a complementiser to make the sentence grammatical. The approach outlined above will use that as a determiner in an ambiguous case such as this.</Paragraph> <Paragraph position="8"> These two simple techniques, word order and agreement, are sufficient to handle all the examples we have just presented. In addition, free text analysis has shown no violations to this approach (Milne 1983). These techniques provide the same coverage as Marcus's diagnostic, with the added bonus that the determiner is attached before the NP is built.</Paragraph> <Paragraph position="9"> That can only be a complementiser when a that S- is expected. Hence the rules using that to start an embedded sentence are only activated when the verb has the feature THAT-COMP. The rules in THAT-COMP will fire when that is followed by something that can start an NP. This ensures that the S- will have a subject and means that that will be taken as a pronoun in the following sentences: (44) I know that hit Mary.</Paragraph> <Paragraph position="10"> (45) I know that will be true.</Paragraph> <Paragraph position="11"> but it will be taken as a complementiser in these sentences: (46) I know that boys are mean.</Paragraph> <Paragraph position="12"> (47) I know that Tom will hit Mary.</Paragraph> <Paragraph position="13"> It seems that, unless the S- has a subject, the pronoun use of that is preferred. Otherwise one would have a complementiser followed by a trace, rather than a unmarked complementiser, followed by a pronoun. This rule provides more complete coverage than Marcus's diagnostic since it examines the second buffer.</Paragraph> <Paragraph position="14"> The rule to handle pronouns in general is of low priority and will only fire after all other uses have failed to match. That is treated in the same way.</Paragraph> <Paragraph position="15"> That will be identified as a relative pronoun only if it occurs after a headnoun and the packet NP-COMPLETE is active. This situation will be handled in the same manner as the usual relative clause rules and will then cover: (48) I know the boy that you saw.</Paragraph> <Paragraph position="16"> (49) I know the boy that hit you.</Paragraph> <Paragraph position="17"> The most difficult case for that is when the verb is subcategorised: VNPS-That is, it can take an NP subject, followed by a that S-. For these examples, ROBIE may have to decide if the series of words following that is a relative clause or an embedded sentence.</Paragraph> <Paragraph position="18"> In the following sentences, the lookahead would have to be more than three buffers. (Brackets indicate words in the buffers. The last word is the disambiguating word.) (50) I told the girl \[that\]\[the\]\[boy\] hit the story (51) I told the girl \[that\]\[the\]\[boy\] will kiss her It can be seen that in these sentences the disambiguating word is outside our three buffers. How do people handle these, and what should our parser do? In Milne (1983) it was shown that when the syntax could not resolve the ambiguity with its two-buffer lookahead, the decision of which interpretation to use might be made using non-syntactic information. It was also stated that if context can affect the interpretation of the sentence, then non-syntactic information is being used to select the Computational Linguistics, Volume 12, Number 1, January-March 1986 7 Robert Milne Resolving Lexical Ambiguity in a Deterministic Parser interpretation. The reader can experiment for himself and see that context does affect the interpretation of these sentences. Therefore it is predicted that non-syntactic information is being used to interpret these sentences, and that this problem should be resolved not on a semantic basis but on a non-syntactic one.</Paragraph> <Paragraph position="19"> This explains why some of these examples cause difficulty and others do not. The psychological evidence from cases using that is scant, and I feel no conclusions can be reached here. My theory predicts that context will strongly affect these examples and, if they are strongly biased to the incorrect reading, a garden path should result.</Paragraph> <Paragraph position="20"> One well-known example in this area is (52): (52) I told the girl that I liked the story.</Paragraph> <Paragraph position="21"> (53) I told the girl whom I liked the story.</Paragraph> <Paragraph position="22"> (54) I told the girl the story that I liked.</Paragraph> <Paragraph position="23"> These examples were tested in Milne (1983). The results suggested that (52) was read faster than the other two examples. Many of the subjects were questioned informally after the experiment about their interpretation of the sentence. All reported only one meaning; the Sreading. None of the subjects said that they noticed the relative clause reading, hence the result. The experiment however, was not designed formally to distinguish these. To handle the examples we have seen in this section, Marcus had four diagnostics, one of which was very complicated. I have just shown how to handle all four cases of that without any special rules, merely substituting enforced agreement and rejecting ungrammatical sentences.</Paragraph> </Section> </Section> <Section position="13" start_page="0" end_page="0" type="metho"> <SectionTitle> 3.7 HANDLING THE WORD HA VE </SectionTitle> <Paragraph position="0"> Let us now look at the elimination of Marcus's HAVE-DIAGNOSTIC in relation to the use of agreement we have been discussing in this section. The problem with have is illustrated by the following sentences: (55) Have the students take the exam.</Paragraph> <Paragraph position="1"> (56) Have the students taken the exam? In these, we must decide if have is an auxiliary verb or a main verb and whether the sentence is a yes-no question or an imperative. The sentences have the same initial string until the final morpheme on take. To handle this case, Marcus (1980:211) used this rule:</Paragraph> </Section> <Section position="14" start_page="0" end_page="0" type="metho"> <SectionTitle> &quot;RULE HAVE-DIAG PRIORITY:5 IN SS-START </SectionTitle> <Paragraph position="0"> \[have,tenseless\]\[np\]\[t\] -~ If 2nd is ns,n3p or 3rd is tenseless then run imperative next else If 3rd is not verb then run yes-no-question next else if not sure, assume it's a y/n-q and run yes-no-question next&quot;.&quot; This rule seems to be necessary in order to distinguish between the question and the imperative. If one tries to ascertain exactly what occurs, the apparent complexity is revealed. Note also that Marcus's rule defaults to a yes-no question twice in this diagnostic. The following sentences illustrate the distinction this rule makes. (57) Have the boy take the exam.</Paragraph> <Paragraph position="1"> (58) Have the boy taken the exam.</Paragraph> <Paragraph position="2"> (59) Have the boys take the exam.</Paragraph> <Paragraph position="3"> (60) Have the boys taken the exam? It can be seen that YES-NO QUESTION should run only when the NP following is plural and the verb has &quot;en&quot; (i.e., taken). \[Only (60) has a plural noun, the boys, and the verb taken.\] This can also be understood as: the sentence is an imperative if the item in the 2nd buffer is not plural and the verb is tenseless. Thus, the first three examples above are Imperatives because either the noun (boy) is singular (57 and 58) or the verb is tenseless (59). The second part of the rule takes care of the fact that the third buffer must contain a verb for the imperative, as this would be the main verb of the embedded sentential object.</Paragraph> <Paragraph position="4"> Let us 10ok more closely at the reason why only (60) is a question. Firstly, if the sentence is a yes-no question, then aux-inversion must occur. When this happens, Have will be adjacent to the verb that was in the third buffer. In order for ROBIE to continue, the verb must have an &quot;en&quot; ending, or have and the next verb will not agree in aspect. This is the basis for discrimination in the earlier examples (57-60).</Paragraph> <Paragraph position="5"> Secondly, in (57) and (58), the noun phrases are singular and both sentences are imperatives. Had the sentence been a yes-no question, have would need to agree with the subject, which must then be plural. Hence, in effect, Marcus's rule checks for number agreement between the subject and verb, and checks that the fixed order of the verb group is obeyed. Let us now look at other situations where this is necessary.</Paragraph> <Paragraph position="6"> PARSIFAL would accept the following ungrammatical strings: (61) *Are the boy running? (62) *Has the boys run? (63) *Has the boy kissing? (64) *Has the boy kiss? For a yes-no question, the inverted auxiliary must agree with the verb after it has been inverted. To stop these ungrammatical constructions, we must enforce verb agreement. The pattern for the rule YES-NO QUESTION should be: \[auxverb\]\[np\]\[verb\], agree(auxverb,verb),agree(verb,np). This constraint enforces agreement of the verb and auxiliary verb and the subject and verb. Again this check is based only on the linguistic features of the buffers. Such a constraint effectively blocks the ungrammatical constructions. (The parser will fail if the auxiliary has been inverted, since the auxiliary will not be parsed.) Also the subject NP must agree with the auxiliary verb, so we can also add &quot;agree(auxverb,np)&quot; to the rule, as</Paragraph> </Section> <Section position="15" start_page="0" end_page="0" type="metho"> <SectionTitle> 8 Computational Linguistics, Volume 12, Number 1, January-March 1986 Robert Milne Resolving Lexical Ambiguity in a Deterministic Parser </SectionTitle> <Paragraph position="0"> we did with the HAVE-DIAGNOSTIC! So, by correcting the yes-no question rule, the HAVE-DIAGNOSTIC is redundant.</Paragraph> <Paragraph position="1"> In this section we have seen that Marcus's HAVE-DIAGNOSTIC can be replaced by merely exploiting agreement. It should be pointed out that although this approach has the same coverage as Marcus's diagnostic, it is wrong in some cases. Milne (1983) has a full discussion.</Paragraph> <Section position="1" start_page="0" end_page="0" type="sub_section"> <SectionTitle> 3.8 PLURAL HEAD NOUNS </SectionTitle> <Paragraph position="0"> There is a class of ambiguities that can be resolved merely by enforcing subject/verb agreement. In this section, we see an example from the class of words with the features noun, verb, final-s (plural). If we have two words that can be a plural noun or a singular verb, we can enumerate four cases. Let us look at these possibilities and see that these cases can be disambiguated by simple rules using subject/verb agreement. The following examples illustrate all the possibilities: (65) The soup pot cover handle screw is red.</Paragraph> <Paragraph position="1"> (66) The soup pot cover handles screw tightly.</Paragraph> <Paragraph position="2"> (67) *The soup pot cover handles screws tightly.</Paragraph> <Paragraph position="3"> {68} The soup pot cover handle screws tightly.</Paragraph> <Paragraph position="4"> {69} The soup pot cover handle screws are red.</Paragraph> <Paragraph position="5"> Each of the words pot, cover, handle, and screw can be either a noun or a verb. The &quot;end of constituent&quot; problem is to find out which word is used as the verb and which words make up the complex headnoun. The possible distributions of the morpheme &quot;s&quot; among two words gives us four cases. We deal with each of these in turn. Case 1: In (65) each noun is singular. For this case all ambiguous words must be nouns and part of the headnoun. Due to subject/verb agreement, a singular noun must match a 3rd person singular (v3s) verb, i.e, one without the letter &quot;s&quot;. This case excludes that possibility since none of the words have an &quot;s&quot; at the end. Hence they must all be nouns.</Paragraph> <Paragraph position="6"> Case 2: In (66) handles is a plural noun and each word before it must be a noun. When a singular noun/verb word follows handles, the word (screw) must be a verb and handles is the last of the headnouns. It is not possible to use handles in this situation as a verb, and screw as a noun because of subject/verb agreement.</Paragraph> <Paragraph position="7"> Case 3: The examples in this case have two consecutive plural nouns as in (67), where both words have noun/verb ambiguity. (Do not confuse plural &quot;s&quot; with possessive &quot;'s&quot;).</Paragraph> <Paragraph position="8"> When the first plural is a noun, then the second one can be a verb only if it is part of a different constituent. Examples of this are the following. (Sentences beginning with &quot;?&quot; are considered grammatical but unacceptable to most readers.) (70) ?The soup pot machine handles screws easily.</Paragraph> <Paragraph position="9"> (71) The soup pot machine handles screw easily.</Paragraph> <Paragraph position="10"> (72) Which years do you have costs figures for? (73) Do you have a count of the number of sales requests and the number of requests filled? \[(72) and (73) are from Martin, Church, and Patil (1981).\] Because there is a non-plural headnoun followed by a plural headnoun, this case is really a subset of Case 4. In general, the problems and issues for Case 4 dominate the resolution of this ambiguity.</Paragraph> <Paragraph position="11"> Case 4: Sentences (68) and (69) both have the same word initial string until after screws, but in (68) screws is a verb while in (69) screws is part of the headnoun. In this situation, where the final word in a series is plural, each word before it must be a noun. The word itself can be either a noun or a verb, depending on what follows. These can be recognised as a pair of potential garden path sentences, as discussed in Milne (1982). Therefore, this is the case to which the Semantic Checking Hypothesis applies and the predictions of Milne (1982) apply. In that paper, the idea of potential garden path sentences is presented. These are sentences that may or may not lead to a garden path. Each garden path sentence has a partner, which is similar but not a garden path. It is proposed that the decision as to how to resolve the ambiguity that may lead to a garden path should be made by semantics and not by syntax. This theory is called the Semantic Checking Hypothesis. For full details see Milne (1983).</Paragraph> <Paragraph position="12"> In this section, we have looked at resolving a simple case of noun/verb ambiguity. In order to resolve this ambiguity, it was necessary merely to exploit agreement between the subject and verb in number and person.</Paragraph> <Paragraph position="13"> Due to number and subject verb agreement, these facts have a linguistic base. They rely on the fact that a final &quot;s&quot; marks a plural noun but a singular verb. If the verb is v3s (verb agrees with a 3rd person, singular noun, as with the &quot;s&quot;), then the subject of the verb must be singular, or else the sentence is ungrammatical. This is why all the words before the v3s word must be nouns. If any of these words were used as a verb, then subject-verb agreement would be violated. This is why (67) is ungrammatical. If the verb is v-3s (agrees with any noun phrase except 3rd person, singular i.e., no &quot;s&quot;), then the subject cannot be singular. (65) has no plural subject and so cannot have a v-3s verb. In (66) handles provides a plural subject, so screw, which is v-3s, can agree.</Paragraph> </Section> </Section> class="xml-element"></Paper>