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<?xml version="1.0" standalone="yes"?> <Paper uid="P90-1025"> <Title>THE LIMITS OF UNIFICATION</Title> <Section position="4" start_page="0" end_page="194" type="metho"> <SectionTitle> 2 The Paradox </SectionTitle> <Paragraph position="0"> A typical paradigm thin is presented to show the almost transparent application of unification to agreement phenomena is the following: (I) a. The sheep is ready.</Paragraph> <Paragraph position="1"> b. The sheep are ready.</Paragraph> <Paragraph position="2"> c. The sheep is there.</Paragraph> <Paragraph position="3"> d. The sheep are there.</Paragraph> <Paragraph position="4"> e. *The sheep that is ready are there. Sentences (la) through (ld) are taken to indicate that &quot;sheep&quot; is underspecified with regard to number;, it can be either singular or plural. (le), on the other hand, shows that &quot;sheep&quot; cannot be both singular and plural at the same time. In the relative clause, &quot;is&quot; is marked as singular, and &quot;sheep&quot;, interpreted as its subject via the relative connector &quot;that&quot;, must also be singular. On the other hand, &quot;are&quot; in the matrix clause is marked as plural, and &quot;sheep&quot;, its subject, must also be plural. Under a unification analysis, these facts are explained in the following way: &quot;sheep&quot; is syntactically unspecified for the feature number. The process of subject-verb agreement is effected by unification. Therefore, when &quot;sheep&quot; appears as the subject of a finite verb, unification will fix its number as singular or plural (unless the finite verb itself is ambiguous). (le) is ungrammatical, then, since the values singular and plural cannot unify and the fact that &quot;sheep&quot; must agree with both &quot;is&quot; and &quot;are&quot; in number would require their unification. This illegal feature configuration is shown in (2).</Paragraph> <Paragraph position="5"> (2) IV \[num:sg\]\] --. \[N \[num:{sg,pl}\]\] ,-- IV \[num:pl\]\] (&quot;is&quot;) (&quot;sheep&quot;) (&quot;are&quot;) Here, the arrows indicate the notional flow of information under agreement, but have no theoretical status. They indicate that agreement between &quot;sheep&quot; and &quot;is&quot; would set &quot;sheep&quot; 's number feature to singular, while agreement with &quot;axe&quot; would set it to plural More generally, the unification approach to agreement rules out the following configuration:</Paragraph> </Section> <Section position="5" start_page="194" end_page="194" type="metho"> <SectionTitle> (3) *\[X \[F:a\]\] -~ \[36 \[F:{a, ;3}\]\] #- \[Z \[F:~\]\] </SectionTitle> <Paragraph position="0"> Here \[F : z\] denotes feature F, with value z, and \[F : {z,g}\] indicates feature F with value either z or V, z and Y distinct. Thus, this schema indicates that a category which is specified for the values a and B for feature F cannot simultaneously agree in this feature with categories that specify distinct values for F. In the rest of this section, I will show cases of constructions which match this schema but are still grammatical.</Paragraph> <Section position="1" start_page="194" end_page="194" type="sub_section"> <SectionTitle> 2.1 Case 1: German Free Relatives </SectionTitle> <Paragraph position="0"> In German, as Groos and van Riemsdijk (1979) demonstrate, free relative clauses require that the relative pronoun agree in Case both with the position of the relafive clause as a whole and also with the position with which the relative pronoun is construed (i.e. with the gap which the relative pronoun fills). This is shown in (4) and (5), where the matrix verb and the verb in the flee relative are annotated with the Case the relative pronoun must bear in that clause.</Paragraph> <Paragraph position="1"> (4) a. Wer nicht stark ist, muss klug sein.</Paragraph> <Paragraph position="2"> who not strong is must clever be</Paragraph> </Section> </Section> <Section position="6" start_page="194" end_page="194" type="metho"> <SectionTitle> ACC ACC ACC </SectionTitle> <Paragraph position="0"> 'I take whomever you recommend to me.' b. *Ich nehme, wen du vertraust.</Paragraph> </Section> <Section position="7" start_page="194" end_page="194" type="metho"> <SectionTitle> ACC ACC DAT </SectionTitle> <Paragraph position="0"> *Ich nehme, were du vertraust.</Paragraph> </Section> <Section position="8" start_page="194" end_page="194" type="metho"> <SectionTitle> ACC DAT DAT </SectionTitle> <Paragraph position="0"> I take who you trust 'I take whomever you trust.' Assuming that &quot;Case assignment&quot; is actually a form of agreement between a verb and a noun phrase that it governs, the data in (4)-(5) seems to fit nicely into a unification approach. However, the neuter fee relative pronoun was, which is both nominative and accusative, can seemingly agree with both nominative and accusative Case assigning elements at the same time: (6) Was du mix gegeben hast, ist p~chtig.</Paragraph> <Paragraph position="1"> What you me given have is wonderful NOM/ACC ACC NOM 'What you have given to me is wonderful.' (7) Ich habe gegessen was noch tlbrig war.</Paragraph> <Paragraph position="2"> I have eaten what still left was</Paragraph> </Section> <Section position="9" start_page="194" end_page="195" type="metho"> <SectionTitle> Ace NOM/ACC NOM </SectionTitle> <Paragraph position="0"> 'I ate what was left.' Note that sentences (6) and (7) are precisely instances of schema (3), just as (le) is. Hence, if the explanation of the ungrammaticality of (le) is correct, we should expect (6) and (7) to be ungrammatical.</Paragraph> <Paragraph position="2"> A possible solution to this seeming paradox, which still uses unification to effect agreement, is the following. 2 Assume that Case in German is not a single-valued feature, but rather an array of the different Cases of the language, each of which takes on one of the values T or NIL. We can then handle the data above with the following feature specifications. (The (a) representations use a &quot;path&quot; notation, consisting of attribute-value pairs, like that in Shieber (1986); while the (b) representations use a term notation, with positional features, like that in Definite Clause Grammars (Pereira and Warren (1980)).) (9) wer: a. \[case: \[nom: T\] \[gen: NIL\] \[dat: NIL\] \[acc: NIL\]\] b. (CASE T NIL NIL NIL) (10) wem: a. \[case: \[nom: NIL\] \[gem NIL\] \[dat: T\] \[acc: NIL\]\] b. (CASE NIL NIL T NIL) (11) wen: a. \[case: \[nom: NIL\] \[gem NIL\] \[dat: NIL\] \[ace: T\]\] b. (CASE NIL NIL NIL T) (12) was: a. \[case: \[nora: T\] \[gem NIL\] \[dat: NIL\] \[acc: T\]\] b. (CASE T NIL NIL T) Assuming that a verb is only specified for the Case it assigns and is unspecified for the others, the Case specifications for verbs that take nominal complements would be: (13) geschaffen,nehme,empfiehlst,gegehen,gegessen: a. \[case: \[ace: T\]\] b. (CASE ?val ?val ?val T) (14) vertraust: a. \[case: \[dat: T\]\] b. (CASE ?val ?val T ?val) Similarly, the Case specification for nominative Case assignment, whether this is a property of syntactic structures or of particular lexical items, would be: (15) a. \[case: \[nom: T\]\] b. (CASE T ?val ?val ?val) This solution works, then, because was, and no other free relative pronouns, specifies the value T for 2This possibility was pointed out to me by Andy Haas. more than one element in its Case array and because verbs and other Case &quot;assigning&quot; elements only specify a value for the Case they &quot;assign&quot;, and for no others. This solution of factoring out seemingly contradictory values for a single feature into values of different features allows us to get around the superficial violation of the schema in (3). However, there axe other constructions which are harder to decompose in this fashion.</Paragraph> <Section position="1" start_page="195" end_page="195" type="sub_section"> <SectionTitle> 2.2 Case 2: Hungarian WH Movement and Topicalization </SectionTitle> <Paragraph position="0"> Let us now turn to a more complicated example, from Hungarian, described in Szamosi (1976). In Hungarian, WH words, like full NPs, are marked as either definite or indefinite. The verb in Hungarian is also marked as definite or indefinite, in agreement with its complement.</Paragraph> <Paragraph position="1"> When the complement is an accusative noun phrase, the definiteness marking on verb and noun phrase is the same.</Paragraph> </Section> </Section> <Section position="10" start_page="195" end_page="195" type="metho"> <SectionTitle> -DEF +DEF </SectionTitle> <Paragraph position="0"> 'A book which he wanted g. *Ez az a k0nyv amelyiket akart this that the book which he-wanted</Paragraph> </Section> <Section position="11" start_page="195" end_page="195" type="metho"> <SectionTitle> +DEF -DEF </SectionTitle> <Paragraph position="0"> h. Ez az a k0nyv amelyiket akarta thisthat thebook which he-wanted</Paragraph> </Section> <Section position="12" start_page="195" end_page="196" type="metho"> <SectionTitle> +DEF +DEF </SectionTitle> <Paragraph position="0"> 'This book is the one which he wanted.' When the complement is a finite clause, the verb bears definite agreement.</Paragraph> <Paragraph position="1"> (17) a. J/lnos akarta, hogy elhozzak egy k0nyvet. John wanted that I-bring a book +DEF +DEF b. *Janos akart, hogy elhozzak egy kOnyvet. John wanted that I-bring a book</Paragraph> </Section> <Section position="13" start_page="196" end_page="196" type="metho"> <SectionTitle> -DEF +DEF </SectionTitle> <Paragraph position="0"> 'John wanted me to bring a book.' Finally, WH phrases and topicalized constituents in Hungarian typically appear immediately preceding the verb; verb and WH word or topic~ized noun phrase must agree in definiteness. 3 From these constraints, it follows that WH phrases and topicalized noun phrases extracted from complement clauses must be marked definite. Since the clausal complement forces the verb to bear definite agreement, and since the WH word or topicalized N-P must agree with the verb in definiteness, the WH word or topicalized NP can only be definite.</Paragraph> <Paragraph position="1"> This is shown in the following examples: b. \[NP \[def.'-\] --, IV \[def.-{+,-}\] ,-- \[c \[def:+\] (&quot;egy kt~nyv&quot;) (&quot;akartam') (&quot;hogy&quot;) Let us consider the consequences of expanding out the definiteness feature into an array of separate values, analogous to the German example. First, this would require the underspecified verb forms to be represented as in (23).</Paragraph> <Paragraph position="2"> (23) akam{u~,akartam: a. \[definiteness: \[definite: T\] \[indefinite: T\]\] b. (DEFINITENESS T T) Next, it would require that the WH pronouns be specified as in (24) and (25): (24) amelyiket: a. \[definiteness: \[definite: T\]\] b. (DEFINITENF~S T ?val) (25) amit (18) Ez az a k/~nyv amelyiket akarta this that thebook which he-wanted that I-bring +DEF +DEF +DEF +DEF 'This is the book which he wanted me to bring.' (19) *Egy k0nyv amit akarta hogy elhozzak.</Paragraph> <Paragraph position="3"> a book which he-wanted that I-bring</Paragraph> </Section> <Section position="14" start_page="196" end_page="196" type="metho"> <SectionTitle> -DEF +DEF +DEF-DEF </SectionTitle> <Paragraph position="0"> 'A book which he wanted me to bring.' However, certain Hungarian verb forms 4 bear an ending which is ambiguous between definite and indefinite. In sentences involving such verbs, the WH word may be indefinite.</Paragraph> </Section> <Section position="15" start_page="196" end_page="197" type="metho"> <SectionTitle> -DEF +DEF +DEF-DEF </SectionTitle> <Paragraph position="0"> 'It was a book that I wanted him to bring.' Once again, the grammatical (20) and (21) match the prohibited schema (3) Cc&quot; = &quot;complementizer&quot;): (22) a. IN \[def.'-\] ---, IV \[def:{+,-}\] ~ \[c \[dd:+\] (&quot;amit&quot;) CakarnCmk&quot;) (&quot;bogy&quot;) 3ThC/ situation is actually somewhat more complex; sC/~ Szamosi (1976) foC/ full details.</Paragraph> <Paragraph position="1"> 4The first person singular past indicative and the first person plural present conditional.</Paragraph> <Paragraph position="2"> hogy elhozzam, a. \[definiteness: \[indefinite: T\]\] b. (DEFINITENESS ?val T) Note that when either of these pronouns appeared with an underspecified definite and indefinite verb, such as those in (23), it would wind up with the definiteness specification in (23). This would totally neutralize the definiteness~ndefiniteness contrast in such cases. But, in fact, no such ambiguity of interpretation is reported: a definite or indefinite WH phrase or topicalized noun phrase that appears in the suitable configuration with one of these ambiguously definite or indefinite verbs is interpreted as uniquely definite or indefinite, as is consistent with its overt maddng, and not as ambiguous between definite and indefinite, as the proposed unification analysis would require. Thus, this unification based solution to a problem of a morphological ambiguity entails an ambiguity of interpretation that is not attested.</Paragraph> <Paragraph position="3"> Moreover, aside from the empirically incorrect predictions about semantic interpretation, there is a more fundamental problem with the unification account of agreement. As was pointed out above, treating agreement as unification implies that structures meeting the schema in (3) should be superficially ungrammatical. In fact, this seems to be universally false: in every case in natural language in which an element does not molphologically distinguish between two or more values of a featureqa situation often referred to as morphological neutralization---it behaves as if this distinction is also neutralized for purposes of agreement. That is, instead of the configuration in (3) being universally ruled out, it is universally attested. This creates a paradox, since the ungrammaticality of (le) seems to depend on the ungrammaticality of structures matching the configuration in (3). To demonstrate that this seeming paradox is supported by the data, in the rest of this section, other examples will be presented to show that the configuration ruled out in (3) recurs again and again across the languages of the world.</Paragraph> <Paragraph position="4"> 2.3 Case 3: Objects of Conjoined VPs In French, as Kayne (1975) points out, it is possible to conjoin past participles following the past auxiliary and a weak Cclitic&quot;) object pronoun which is the common object of the conjoined participles, under the requirement that the verbs of the conjuncts assign the pronoun the same Case. This is shown in (26) and (27): (26) Paul l'a insuR~ et mis ~t la porte.</Paragraph> <Paragraph position="5"> Paul him-hasinsulted andput to the door</Paragraph> </Section> <Section position="16" start_page="197" end_page="197" type="metho"> <SectionTitle> ACC ACC ACC </SectionTitle> <Paragraph position="0"> 'Paul insulted him and threw him out.' (27) *Paul l'a frapp~ et dounC/ des coups de pied. Paul him-hasstmck andgiven blows of foot</Paragraph> </Section> <Section position="17" start_page="197" end_page="197" type="metho"> <SectionTitle> ACC ACC DAT </SectionTitle> <Paragraph position="0"> 'Paul struck him and gave him some kicks.' However, once again, if the object pronoun is marked for more than one Case, the conjunction of par. ticiples assigning those Cases is allowed.</Paragraph> <Paragraph position="1"> b. Harm stal og bor0aOi kOku.</Paragraph> <Paragraph position="2"> he stole and ate a cookie</Paragraph> </Section> <Section position="18" start_page="197" end_page="197" type="metho"> <SectionTitle> ACC DAT ACC/DAT </SectionTitle> <Paragraph position="0"> 'He stole and ate a cookie.' And German also has similar data, as Pullum and Zwicky (1986) show: (31) a. *Sie findet und hilft Manner/M~nem. she finds and helps men</Paragraph> </Section> <Section position="19" start_page="197" end_page="197" type="metho"> <SectionTitle> ACC DAT ACC DAT </SectionTitle> <Paragraph position="0"> 'She finds and helps men.' b. Er findet und hilft Frauen.</Paragraph> <Paragraph position="1"> he finds and helps women</Paragraph> </Section> <Section position="20" start_page="197" end_page="197" type="metho"> <SectionTitle> ACC DAT ACC/DAT </SectionTitle> <Paragraph position="0"> 'He finds and helps women.' The French, Icelandic, and German examples fall into the now familiar configuration.</Paragraph> <Paragraph position="1"> (32) \[V \[case:A\]\] --~ \[NP \[case:{A,D}\]\] ,-- \[V \[case:D\]\] 2.4 Case 4: Elided Verbs in German Eisenberg (1973) points out that in conjoined German subordinate clauses, the verb in all the non-final clauses can be elided, under identity of person and number agreement.</Paragraph> <Paragraph position="2"> (33) ...weft Hans Bier und Franz Milch trinkt. because Hans beer and Franz milk drinks 3rd 3rd 3rd (28) Paul nous a frapp~ et Paul us has struck andgiven blows ACC/DAT ACC DAT 'Paul struck us and gave us some kicks.' (29) On salt que la police t'a frapl~ one knows that thepolice you-has struck</Paragraph> </Section> <Section position="21" start_page="197" end_page="198" type="metho"> <SectionTitle> ACC/DAT ACC </SectionTitle> <Paragraph position="0"> et donn~ des coups de pied.</Paragraph> <Paragraph position="1"> andgiven blows of foot DAT 'Everybody knows that the police struck you and gave you some kicks.' donnd des coups de pied. '...because Hans drinks beer and Franz, Milk.' of foot (34) *...weft ich Bier und du Milch trinkst/trinke. becauseI beerandyou milk drinks 1st 2nd 2nd 1st '...because I drink beer and you, Milk.' Similar facts hold for Icelandic, as well, as Zaenen and Karttunen (1984) point out.</Paragraph> <Paragraph position="2"> However, in forms which neutralize the person marking on the verb, elision is fine: (30) a. *Harm stal og bor0a0i k6kuna/k0kunni. he stole and ate the cookie ACC DAT ACC DAT 'He stole and ate the cookie.' (35) ...weft wir das Hans und die Muellers den Garten kaufet</Paragraph> </Section> <Section position="22" start_page="198" end_page="201" type="metho"> <SectionTitle> 3 Resolving the Paradox </SectionTitle> <Paragraph position="0"> The previous section presented a paradox. There seems to be evidence, in the form of ungramatical utterances such as (le), that the configuration in (3) is ungrammatical. However, the rest of the section presented evidence from different constructions and different languages which strongly indicates that (3) is the standard agreement configuration throughout the languages of the world. In this section, I will resolve this paradox by proposing that agreement is not effected by unification and but rather by a test for non-distinctness of feature values.</Paragraph> <Paragraph position="1"> 3,1 Neutralization versus Ambiguity First, let us return to example (le), repeated here for convenience (38) *The sheep that is ready are there.</Paragraph> <Paragraph position="2"> Recall that the analysis of this utterance which argued for the ungrammaticality of configuration (3) was based on the assumption that &quot;sheep&quot; is unspecified or under-specified for number. Note that this analysis is tenable if syntactic features alone are considered: syntactically, it seems plausible that &quot;sheep&quot; either has no number feature or that it has a variable, rather than a constant, as the value of this feature. However, when the ramifications of this analysis for semantics are considered, it becomes less tenable: while syntactic frameworks have been constructed in which features can take on under-specified values, most semantic frameworks require features such as singular/plural to be fully specified. That is, semantically, &quot;sheep&quot; can denote an individual or a set of individuals 5 but it cannot denote something indeterminate. This suggests that &quot;sheep&quot; is not underspeC/ified, or vague, but rather ambiguous. That is, there is not a single representation for &quot;sheep&quot;, which is underspecified for number, but rather two distinct entries, fully specified for number in both its syntactic and semantic aspects. If this is the case, the reason that (38) is ungrammatical is not that unification has filled in the underspecified value for number, but rather that subject-verb agreement disambiguates which of the two senses of &quot;sheep&quot; has been encountered and once one of 5Nothing in the present argument hinges on this being the correct Ueatrnent of the singular/plural distinction. It does not matter which of the various proposals about the semantic interpretation of number is chosen. All that matters is that semantic theories require that singular and plural have different denotations, and do not allow indeterminate representations.</Paragraph> <Paragraph position="3"> the fully specified entries is chosen, it naturally cannot agree with a constituent which bears a distinct number.</Paragraph> <Paragraph position="4"> Once utterances like (38) are analyzed as not matching the agreement configuration in (3), it is possible to handle all the cases of morphological neutralization discussed in the previous section. Note that the feature involved in each example of neutralization discussed--Case in German and French, and definiteness on verbs in Hungarian---is either inherently formal, without semantic content (Case) or a feature that does not have any semantic ramification for the category in which it is neutralized: definiteness does affect the interpretation of noun phrases, but it serves purely as a formal agreement marker on verbs. If this observation is correct, then the solution to the apparent paradox runs along the following lines: * Syntactic features which have semantic ramifications, such as number on nouns, tense on verbs, degree on adjectives, are never neutralized (underspecified). They are always fully specified and items which seem to be underspecified with regard to them are, in fact, ambiguous items with distinct, fully specified representations. (But see the discussion in the Section 4.) * Purely formal syntactic features, on the other hand, can be neutralized, producing truly underspecified representations, either through the use of value disjunction or through the use of a variable, rather than a constant, as a feature value. 6 * Agreement is effected not by unification but rather by a non.distinctness check.</Paragraph> <Paragraph position="5"> That is, since we can view unification as logically composed of two parts---variable checking and variable substitution---agreement should be analyzed as involving only variable-matching, but not variable substitution. This would explain why constituents that neutralize a syntactic feature distinction are universally able to behave as if they are simultaneously marked for all the values of the feature that they neutralize: since agreement only involves variable matching, but not variable substitution, the original, underspecified representation is always available for agreement.</Paragraph> <Paragraph position="6"> To make this proposal clearer, I will present an analysis of the German and Hungarian facts, 7 using a term structure type notation, and adding the :OR operator, which introduces disjunctions of variable-free These representations are matched by a non-distinctness check, which performs the same tests as unification. However, the non-distinctness check differs from unification in what it returns. Unification, when applied to two expressions, typically returns either a distinguished symbol, such as Fail, if they do not unify, or a single substitution expression, which is the most general unifier of its input; see e.g. Pereira and Shieber (1987, pp. 63-64). When two expressions are identical, this substitution expression is empty, since no substitutions need to be performed. In this case, then, unification effectively leaves its input unchanged. Thus, unification can be viewed as returning a single indicator of failure and an unbounded set of substitution expressions. Non-distinctness checking, on the other hand, returns a single indicator of failure but also a single indicator of success--an empty substitution expression.</Paragraph> <Paragraph position="7"> Alternatively, non-distinctness checking may be viewing as determining that two expressions are unifiable, without actually unifying them.</Paragraph> <Paragraph position="8"> The following table contrasts the behavior of unification (U) and non-distinctness (~): property of structures instead; however, the Case specification of the appropriate structure would be the same as here. A similar consideration holds for Hungarian, where the property of a direct sentential complement triggering definite verb agreement might be either a lexical or structural property.</Paragraph> <Paragraph position="9"> SFor a full discussion of the issues involved in adding disjunction to complex-fealm'e based formalisms, see Karttamen (1984), Kasper and Rounds (1986), Kasper (1987), and Iohnson (1989).</Paragraph> <Paragraph position="10"> Case: 1. z, y are variable-free and non-disjunctive: a. z=y b. otherwise: 2. ~, y contain variables but are non-disjunctive: a. 3MGU(z, y) b. otherwise: 3. ;~, y are both disjunctions: a. zny40 b. otherwise: z is a disjunction: a. y is a term in b. otherwise: where MGU(z, y) is the most general unifier of z, y; NIL is the empty substitution expression; and (a ,--- t) indicates a substitution expression in which fl substitutes for a. 9 In examples (4) and (5) in German, and (16)-(19) in Hungarian, clause 1 applies. Since the terms involved in the agreements in all these examples are variable-fi'ee, the results are identical under the unification and non-distincmess analyses. In the German examples (6) and (7), which involve was, clauses 4 and 5 are the relevant ones, and it is here that the difference between the unification approach to agreement and the non-distinctness approach is apparent. Under the unification approach, once the disjunctive Case feature value associated with was unifies with a fully specified Case feature, a substitution list is produced that replaces the disjunction with one of its values: (39) (:or (nom) (arc)) U (nom) ((:or (nom) (arc)),--(nom)) On the other hand, the non-distincmess check returns a null subsitution, so that the disjunction remains, allowing the Case feature of was to agree with distinct values of Case on different applications of non-distinctness. (40) a. (:or (nom) (acc)) ~ (nom)) =~ NIL b. (:or (nom) (acc)) ~ (arc)) => NIL 9Note that this is an extension of the standard conception of substitution&quot; in systems without disjunction, in which a term substitutes for a variable, but not for a variable-free term. However, the addition of disjunction requires such an extension.</Paragraph> <Paragraph position="11"> A similar analysis holds for examples (20) and (21) in Hungarian; in these cases, it is clause 2 which is relevant.</Paragraph> <Paragraph position="12"> The treatment of the conjoined verb phrase facts in Section 2.3 is analogous to that of the cases already discussed. However, one point is worth discussing here. It has not yet been made clear how it is that the object of the conjoined verb phrase is able to agree separately with each verb in the conjunct. While it might be possible to handle this mechanically by postulating some special percolation rule that combines the features of the conjuncts together into some underspecified or disjunctive form, there is a much more straightforward solution, namely, to postulate that the examples in Section 2.3 are generated by ellipsis. Certainly, given the strong lexical thrust of recent grammatical frameworks, in which syntactic structures, such as verbal complement structures, are projected from lexical representations, it is hard to see how such examples could not be analyzed as cases of ellipsis, at least in conligurational languages. Thus, example (28) would be analyzed as in (42) rather than (41).</Paragraph> <Paragraph position="13"> (41) nous a \[vP \[vp frappe\] et \[vP donn~ des coups de pied\]\] (42) \[vP \[vP nous a frapp4\] et \[vF, \[xP e\] \[v e\] donn4 des coups de pied\]\] In non-configurational languages, since complements may not be localized in any fixed position, some other mechanism for associating a head with its complements is needed, independent of these neutralization facts. In an active-objects approach to syntax, such as that outlined in Ingria and Pustejovsky (1990), message-passing would be the logical way of associating a head with its complements and would extend to the conjunction cases, as well. In any event, whatever mechanism is operative in the non-conjunctive case should also apply to the conjoined case.</Paragraph> <Section position="1" start_page="200" end_page="201" type="sub_section"> <SectionTitle> 3.2 Related Work </SectionTitle> <Paragraph position="0"> This paper is not the first to consider the problem that neutralization facts pose for theories of agreement. In particular, Zaenen and Karttunen (1984) and Pullum and Zwicky (1986) consider data of the type presented in Sections 2.3 and 2.4. However, the analysis of agreement proposed here seems more general in a number of ways. 1deg loin all fairness, Zaenen and Karttunen and Pullum and Zwicky also consider aspects of conjunction and agreement that fall outside the scope of the present paper.</Paragraph> <Paragraph position="1"> While the earlier analyses only considered neutralization in the context of conjoined structures, as in Sections 2.3 and 2.4, this paper has examined the problem in general. In particular, the solutions proposed by Zaenen and KartOmen and Pullum and Zwicky crucially depend on the neutralized item standing in an agreement relation with a conjunction and, hence, cannot extend to cases of neutralization that do not involve conjunction.</Paragraph> <Paragraph position="2"> While Zaenen and Karttunen and Pullum and Zwicky agree with the present analysis in associating the neutralized constisment with each conjunct of the conjunction directly, rather than through the conjunction as a whole, both of their analyses require this association to be stated as a separate principle. If the brief sketch presented at the end of the preceding section is correct, no such stipulation is necessary. Rather, the behavior of neutralization with respect to conjunction follows from the interaction of the general agreement procedure with the way in which heads are associated with their complements.</Paragraph> <Paragraph position="3"> Zaenen and Karttunen leave the bulk of the question of what features can be neutralized as a research topic. Pullum and Zwicky, on the other hand, limit neutralization to those features imposed by agreement.</Paragraph> <Paragraph position="4"> This is essentially the position argued for here, although there are subtle differences between the two proposals and some problematic data (which we will return to in Section 4). However, this proposal does seem to be fundamentally correct, and, combined with the view of agreement as non-distinctness, yields a more empiricany valid theory of agreement than one which equates unification with agreement or which limits the effects of neuWalization to conjoined structures.</Paragraph> <Paragraph position="5"> Moreover, this view of agreement should contribute to the portability of natural language systems across languages. While it might be possible to reconcile the type of agreement behavior discussed here with a forrealism in which unification is used for agreement by the use of arrays of feature values or some even more byzantine mechanism, such an approach would increase the fragility of any system embodying it. In a theory such as the one here, it should be possible to distinguish cases of ambiguity firom cases of neutralization straightforwardly and to assign the appropriate representation accordingly. In a system that tried to maintain the use of unification for agreement by means of elaborated representations, the designer of a grammar for a new language would be faced with the problem of either using the elaborated representation for all cases of morphological underspecification, and, perhaps, blowing up the size and complexity of the grammar, or reserving the elaborated representation for just those forms which enter into an agreement relation. This would require a thorough study of all the morphological forms of the language and the constructions they enter into before feature structures could be designed and might entail large scale changes later if previously unnoticed cases of neutralization were discovered.</Paragraph> </Section> <Section position="2" start_page="201" end_page="201" type="sub_section"> <SectionTitle> 3.3 The Place of Unification in Grammar </SectionTitle> <Paragraph position="0"> The proposal that agreement is not effected by unification does not, however, mean that unification plays no role in grammar. On the contrary: in most complex-feature based systems, semantic features axe also full-fledged parts of syntactic representations and unification is used to build up more complex terms out of simpler or less specified terms and to build up formulas out of terms, n There is no argument at all in the data presented here that unification does not continue to play this role. In fact, there is a certain historical niceness in the picture of grammar that has been developed here: variable matching (non-distinctness) is used to effect agreement, and variable substitution (unification) is used to build up semantic representations.</Paragraph> <Paragraph position="1"> The reason why this view is historically satisfying is that it corresponds to views of agreement and semantic interpretation that were independently developed in theoretical and computational linguistics. In the earliest forms of generative grammar, it was recognized that certain constructions, such as the various types of ellipsis, depended on a notion of identity. Over the years, this notion of identity was refined into one of non-distinctness. Two linguistic representations agree if they are non-distinct from one another;, they do not need to be identical (see Chomsky (1965, p. 181)).</Paragraph> <Paragraph position="2"> The view of agreement presented here accords with this well-established view. The use of unification for building up semantic representations, in turn, is based on Robinson's (1965) work on resolution theorem proving.</Paragraph> <Paragraph position="3"> Thus, using unification to build up semantic representat'ions, but not for agreement, returns it to something close to its original use.</Paragraph> <Paragraph position="4"> There are two other places where unification may play a role in grammar, although other mechanisms are also possibile in these cases. The first is feature percolation and the second is the use of empty categories, such as traces. Whereas agreement has been used here to mean matching of features between sister nodes, typi11SC/e Percira and Warren (1980), Shieber (1986), and Pereira and Shieber (1987) for more detailed discussion of semantic interpretation in complex-feature based grammars.</Paragraph> <Paragraph position="5"> cally of distinct categories, feature-percolation involves the matching of features between one constituent and a constituent which it dominates, where the dominating constituent is a projection of the dominated, in the sense of the X-Bar theory of phrase structure (Choresky (1970), lackendoff (1977)). For example, a noun phrase has the same person and number features as its head noun, a verb phrase, the tense and mood of its head verb, etc. Unification has typically been used to effect feature-percolation and nothing in the data presented here suggests that it is wrong to use unification for this purpose. And while the proposal that agreement and feature-percolation are handled by different mechanisms is not usual in complex-feature based grammars, it is also not unprecedented. Ross's (1981) Local Grammar formalism is a complex-feature based grammar in which feature percolation and agreement are distinct.</Paragraph> <Paragraph position="6"> Finally, unification has been used to &quot;pass&quot; features between a &quot;dislocated&quot; element and its trace. Here again, unification remains a viable mechanism. However, there are alternatives mechanisms for both these functions, such as inheritance and delegation, whose use should probably be investigated.</Paragraph> </Section> </Section> <Section position="23" start_page="201" end_page="202" type="metho"> <SectionTitle> 4 Future Research </SectionTitle> <Paragraph position="0"> There are a number of theoretical and practical issues that the analysis presented here raises. Their discussion will conclude this paper.</Paragraph> <Paragraph position="1"> First of all, there is the question of how the non-distinctness test for agreement can be incorporated into a system in which unification is used for semantic interpretation and other purposes. Since non-distinctness returns a subset of the values returned by unification, interaction between non-distinctness and unification should be straightforward. However, a system using both these mechanisms would also need to contain some method for specifying which features of which constituems are subject to unification and which are subject to non-distinctness. This suggests the necessity of some sort of type declaration system, in which features are declared as semantically relevant or not for a particular category. The BBN ACFG formalism (Boisen 1989a,b), a form of Definite Clause Grammar, already includes a type declaration system, which has proven very useful for maintaining the consistency of large grammars. It should be possible to extend this kind of type system to the degree of delicacy required by a system incorporating both unification and nondistinctness. null A more problematic issue is the exact specification of the features which can be neutralized and those which can be ambiguous, and their contexts. In Section 3.1, it was suggested that semantically relevant features enter into ambiguity relations, while all others produce neutralization. However, the notion of semantic relevance may need to be refined. Zaenen and Kartunnen (1984) produce examples such as the following: (43) der Antrag des oder der Dozenten the petition the or the docent(s)</Paragraph> </Section> <Section position="24" start_page="202" end_page="202" type="metho"> <SectionTitle> SG PL GEN-SG/GEN-PL </SectionTitle> <Paragraph position="0"> 'the petition of the docent or docents' (44) *Ich have den Dozenten gesehen und geholfen. I have the docent(s) seen and helped</Paragraph> </Section> class="xml-element"></Paper>