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<?xml version="1.0" standalone="yes"?> <Paper uid="C92-1031"> <Title>THE PROPER TREATMENT OF WORD ORDER IN HPSG*</Title> <Section position="1" start_page="0" end_page="0" type="metho"> <SectionTitle> THE PROPER TREATMENT OF WORD ORDER IN HPSG* EAREL OL~VA </SectionTitle> <Paragraph position="0"/> </Section> <Section position="2" start_page="0" end_page="6" type="metho"> <SectionTitle> Abstract: </SectionTitle> <Paragraph position="0"> This paper describes a possibility of expressing ordering constraints among non-sister constituents in binary branching syntactic structures on a local basis, supported by viewing the binary branching structure as a list (rather than a tree) of constituents within HPSG-style grammars. The core idea of such a description of ordering is constituted by creating a type lattice for lists. The possibilities of expressing different approaches to word order in the framework are briefly discussed, exemplified and compared to other methods.</Paragraph> <Paragraph position="1"> In the standard immediate-constituent based approaches, the &quot;free&quot; word order I is described either directly in the phrase-structure (PS) rules, which thus express simultaneously both dominance (mother/daughter) relations and precedence (ordering) relations between syntactic categories, or, in more recent formalisms such as GPSG or HPSG, by the linear precedence (LP) rules creating a separate component of the grammar, whose other component is the set of immediate-dominance (ID) rules. In both these cases, the ordering constraints are limited to sister constituents, i.e.</Paragraph> <Paragraph position="2"> they are strictly local.</Paragraph> <Paragraph position="3"> One of the problems of both these variations of the standard PSapproach is the description of adjuncts (free modifiers). On the usual assumption that their number per clause is in principle not limited (though finite for a particular clause), an approach to their ordering presupposing them to be all sister constituents laust necessarily presuppose also an (at least potentially) infinite set of generative rules (e.g., a set induced by a Kleene star used in a &quot;basic&quot; variant of one of the rules).</Paragraph> <Paragraph position="4"> In languages whose grammar allows for more word order freedom than English 2, it is often the case that adjuncts and complements of a head (typically, but not solely, of a finite verb) can be freely intermixed, which makes the approach where the locality of LP constraints forces the head as well as its modifiers (both complements and adjuncts) be expanded as sisters still less attractive.</Paragraph> <Paragraph position="5"> Another possibility of description of word order is the &quot;topological&quot; approach used predominantly in more traditionally oriented German linguistics. Applied to German, this approach divides a clause into several word order &quot;fields&quot; (&quot;Vorfeld&quot;, &quot;Mittelfeld&quot;, &quot;Nachfeld&quot;, &quot;linke/rechte Satzklammer&quot;) whose mutual position is fixed, and studies mainly the word order regularities within these &quot;fields&quot;. Though a lot of work has been done and many valuable insights presented within this paradigm, seen from the viewpoint of computational linguistics this approach has the fatal disadvantage that it is extremely difficult to formalize within the standard frameworks (e.g., none of the &quot;fields&quot; with the possible exception of &quot;Vorfeld&quot; creates a constituent in any usual sense etc.).</Paragraph> <Paragraph position="6"> AS an alternative to the two basic approaches mentioned above (as a modification of the first one, in fact), the description based on binary right-branching structures has been proposed independently in several works concerned with languages ex-Aca~s DE COL1NG-92, NANTES, 23-28 AO~'T 1992 1 8 4 PROC. OF COLING-92, NnbrrES, AUG. 23-28, 1992 hibiting a considerable share of free word order: in (Uszkoreit,1986) for the description of verb-final clauses Jn German, in (Gunji, 1987) for Japanese and in (Avgustinova and Ol/va,1990) for (mainly) Slavic languages. However, the price paid for the removal of some problems, mainly, the free intermixing of heads, complements and adjuncts, of the above-mentioned more standard descriptions is rather high - at least two problematic points arise due to the strict binarity of the structure. The first of them is the fact that in binary structures no LP-rules relying on the relation &quot;being sister constituent&quot; can be used for ordering heads, complements and adjuncts in cases this is required, since these are not sisters any more. The second problematic point can be seen at best at the variant of the formalism given in (Avgustinova and Oliva, 1990) the occurrence of the phonologically empty rightmost element of the branching 3 (cf. the structure (1) for the string &quot;John kissed Mary yesterday&quot;).</Paragraph> <Paragraph position="7"> The former problem concerning word order is in the majority of the binary-branching approaches (as far as they are at all concerned with it) solved by introducing word order mechanisms which are either of non-local nature or which burden the syntactic categories (understood as feature bundles) with otherwise unmotivated features used solely for the purpose of imposing ordering constraints (and most often, with a combination of the two).</Paragraph> <Paragraph position="8"> Neither approach is more fortunate than the other - nonulocality is surely an unwished phenomenon in the description, and the presence of special ordering features in the categories is hardly better, i.a. also because order is a property of the syntactic structure (made of categories) rather than of the categories themselves 4 .</Paragraph> <Paragraph position="9"> This paper will try to show that in spite of the abovementioned reservations the &quot;binary branching&quot; can be a correct and fruitful approach to syntactic description if seen from a slightly different viewpoint. In order to get the proper perspective, let us observe the &quot;binary branching&quot; structure for the example sentence &quot;The small boy ate an apple&quot; shown in (2) .</Paragraph> <Paragraph position="10"> There are several things to be taken into consideration here.</Paragraph> <Paragraph position="11"> The most obvious among them is the division of the structure into &quot;levels&quot; - contiguous sequences.of nodes with identical marking. Thus one</Paragraph> <Paragraph position="13"> &quot;VP-level&quot; and two &quot;NP-levels&quot; are to be clearly seen, each having a distinguished element at its end (the phonologically empty element).</Paragraph> <Paragraph position="14"> Further, it can be observed that each of the &quot;levels&quot; has one (and only one) other distinguished element somewhere in a non-final position - it is the V element for the &quot;VP-level&quot; and the N elements for the &quot;NP-levels&quot;, in other words each level has a head, It is also worth remarking that the levels of the binary branching have a direct relation to more usual approaches. Thus, the standard i~ediate-constituent tree (3) for the sentence from (2) can be obtained by factorizing (collapsing) the NP nodes from the respective NP-levels into a single one, and by factorizing all the VP-nodes except for the uppermost &quot;sentential&quot; one.</Paragraph> <Paragraph position="16"> the small boy ate Art N i 1 an apple The dependency tree (4) of the same sentence can be then obtained by collapsing all nodes of a level plus its head into a single node.</Paragraph> <Paragraph position="17"> (4) /.a~t e the small The most striking observation concerning the syntactic structures of the kind exemplified in (2), however, is the nature of their data type: showing a strict binary right-directed branching and having a distinguished (by its phonological emptiness) node as their final element, they are in fact nothing but lists 5 .</Paragraph> <Paragraph position="18"> Adopting this view brings along several advantages: - first, the syntactic structure is strictly uniform - and also simpler than the general tree structure, with all (mainly practical) consequences following from this second, the overall usage of lists (whose members may be lists again) brings back the notion of locality of syntactic description - each list used in the structure (i.e., each &quot;level&quot; of the structure as discussed above) constitutes a local domain, creating thus also a natural area of application of local constraints (such as subcategorization, linear precedence etc.) - third, such an approach allows for merging both the syntactic and the topological approaches in a single formal description, keeping, however, the two components clearly separated the categorial information being expressed by means of (syntactic and other) features and their bundles (attribute-value matrices), the topological information being expressed by means of refinement of kinds of lists and their elements and sublists.</Paragraph> <Paragraph position="19"> Thus, given fairly usual assumptions about the nature and function of constituents in a phrase, the general type 6 for nonempty lists from (5) is to be split into subtypes shown in (6) (where minor covers constituents made of cemplementizers, particles etc.).</Paragraph> <Paragraph position="20"> In practice, even more delicate division is needed according to kinds of phrases used and according to the nature of modifiers these phrases allow. Introduction of more fine-grained sub-types may be needed also for the final element of a list (usually nPS1); the respective subtypes should mirror the kinds of phrases used as functions of the &quot;levels&quot; of the syntactic structure, giving thus rise, e.g., to types end ofnp, end ofvp etc.</Paragraph> <Paragraph position="21"> \[\]sing a different form of structural representation enforces also using different form (but not different background intuitions) of rules and principles of the grammar, all of them corresponding tothe types of lists as introduced in the immediately preceding text.</Paragraph> <Paragraph position="22"> Thus, the Head Feature Principle (HFP) is to be expressed as a conjunction of two implications 7 (rather than a single implication, combining conjunctively with other principles of the grammar), one describing the case where the first element of the respective nonempty 8 list is the head of the respective phrase (&quot;level&quot;), the other one describing the rest of the cases. Assuming the version of HPSG using sets (rather than lists) as values of the feature subcat, the Subcategorization Principle has to consist of four implications, each for a particular configuration in the syntactic list 9 . The first part describes the impact of an expansion of a complement, the second the impact of an expansion of a head (consisting just in copying the subcategorization of the head into a special head feature head subcat, with the aim of inheriting the information about the subcategorization of the head constituent into the final element of the respective list via the HFP), the third the impact of an expansion of an adjunct or of a minor category, and the last one expresses the requirement that the subcahegorization of the final element of the phrase, covering no syntactic material, be equal to the subcategorization of the head of the phrase. (The effect of the second and the fourth implication taken together is worth comparing with the above-mentioned condition from works by Uszkoreit and Gunji, namely that the verb - the source of the subcategorization stand at the end of the Assuming further a phonological principle stating that the phonology of constituents of the type nil (and of all its subtypes) is empty while phonology of all other constituents is the combination Ideg of phonologies of their first and rest subconstituents, this approach allows for reduction of the number of grammar schemata (&quot;rules&quot;) describing the eategorial structure to one (similarly as in Gunji, 1987) having the gross shape shown in (9).</Paragraph> <Paragraph position="23"> (9) Ffirst: \[\]\]</Paragraph> <Paragraph position="25"> The word order constraints, on the other hand, can be expressed within the hierarchy of sorts of lists used in the system, by means of which the ordering information is not only kept separated from the categorJal one, but is also formulated in local domains (each constituted by a llst) only.</Paragraph> <Paragraph position="26"> AcrEs DF, COLING-92, NA~rrES. 23-28 hotrr 1992 1 8 7 PROC. OF COLING-92, NANTm% AU~. 23-28. 1992 The practical usage of the idea of using the sort hierarchy of lists for the purpose of expressing word order constraints will be now illustrated on an example. In this example, the symbol &quot;==&quot; will be used for defining the type hierarchy. The type standing on the left-hand side of the &quot;~' will be a supertype of the type standing on its right-hand side. As (i0) \[clause \] == \[verb first_clause \] v \[verb second clause \] v \[verblast clause \] the example proper, let us take a slightly simplified system of German word order as used in the &quot;field&quot;-based approach, and let us assume for the moment that the sorts finite verb, nonfiniteverb, complement and adjunct are primitives (though, obviously, in reality they are net). The description of the word order of the clause then may look like as shown in (10).</Paragraph> <Paragraph position="27"> The first definition in (i0) describes the fact that a clause is either a verb-first clause, a verb-second clause or a verb-last clause. The next three definitions describe the word c rder within these kinds of clauses .</Paragraph> <Paragraph position="28"> The definitions of verb-first and verb-second clauses are quite simple, specifying only the types of the first and rest features of the respective syntactic lists. The definition of verb-last clauses expresses the fact that they can consist either of a verbal modifier followed by (the rest of the) verb-last clause, or of a finite verb, which cannot be followed by any syntactic material 11 . The last four definitions express actually the &quot;field&quot; approach to the German sentence. The first of them states that the forefield consists of a verbal modifier, which, in turn, is defined as being either a complement or an adjunct (in the last definition). The specification of middle field (and contingent following parts of the clause) says that the it can contain first of all any verbal modifi-ACRES DE COLING-92, NANTKS, 23-28 Aotrr 1992 1 8 8 PROC. OF COLING-92, NANTES. AUG. 23-28, 1992 er followed by (the rest of the) middle field or that it can be empty or that it can contain a nonfinite verb followed by an afterfield. Finally, the afterfield is defined either as empty or as containing a verbal modifier followed by (the rest of the) afterfield. Some clarification of the general idea should be brought in by the structure (12) for the sentence (Ii). Here, as well as in the structures that follow, only the most specific deducible sorts are given for a type (e.g., with the constituent Hans the sort con~lement is used rather than the sort forefield, because co~plement is more specific than forefield). null (ii) Hans hat gestern Maria ein Buch gegeben. The previous example showed a relatively simple case where the number of elements (&quot;fields&quot; of the clause) to be ordered was low and more or less given in advance, and their ordering absolute (e.g., forefield first, finite verb second, middlefield third etc.). However, the descriptive power of the approach is not limited to this: cases where the number of elements is not given beforehand and their ordering is not absolute can be coped with, too, as well as cases of word order combining the two kinds of requirements. For more details see (Oliva, 1992).</Paragraph> <Paragraph position="29"> The principal achievement of the approach presented is the (re)introduction of locality into binary branching structures allowing for replacing the more standard but in this case unsuitable concept of ordering constraints holding for sister constituents by a very similar concept of ordering constraints holding for a list of constituents. However, the introduction of lists allows also for easiness of expressing different other techniques of describing word order arld its variations, such as the &quot;topological&quot; approach discussed above 12 or the &quot;systemic ordering&quot; as worked out by Prague linguists (e.g., Sgall et ai.,1986) etc. Worth consideration is also the relation of this approach based on typing of lists of surface constituents (expressing thus i.a.</Paragraph> <Paragraph position="30"> also their obliqueness hierarchy) to the &quot;<<&quot;-type of LP-rules of the standard HPSG, which, unlike the approach discussed, force obliqueness of complements to be expressed repeatedly within the subcategorization list of each head.</Paragraph> <Paragraph position="31"> The applicability of tile method of description of word order as discussed in this paper has been proved by using it successfully in an experimental grammar of German developed in the STUF '91 forlaalism within the iPSLog project.</Paragraph> </Section> <Section position="3" start_page="6" end_page="6" type="metho"> <SectionTitle> Acknowledgement </SectionTitle> <Paragraph position="0"> I am thankful to Taxi/a Avgust inova, Judith Engelkamp, Gl:egor Erbach, Eva Haji~ovA and Petr Sg,~!\] for most valuable corm~ents on the first draft of\[ this papel .</Paragraph> <Paragraph position="1"> AcrEs :OE COLlNG-92. NANTES. 23-28 AoG7 1992 1 8 9 PROC. OF COLING-92, NAN IT, S, AUG. 23-28. 1992 Footnotes: * Needless to say, hopefully, that this title should not be taken all that seriously ...</Paragraph> <Paragraph position="2"> 1 In fact, free constituent order - but I shall stick to the traditional terminology in this paper.</Paragraph> <Paragraph position="3"> 2 Though even the English word order is not that rigorous as is often assumed especially as to the position of different adjuncts.</Paragraph> <Paragraph position="4"> not latent also in the approaches presented by Uszkoreit and Gunji - they just use the clause-final position of the verb in German and Japanese (i.e., a phenomenon from the fixed-word-order sphere) which helps to cover it.</Paragraph> </Section> class="xml-element"></Paper>