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<Paper uid="C96-1029">
  <Title>mentals. Center for the Study of Language and Information</Title>
  <Section position="2" start_page="163" end_page="166" type="metho">
    <SectionTitle>
2 An alternative proposal
</SectionTitle>
    <Paragraph position="0"> In this paper we explore an alternative to horizontal relatedness which exploits the idea that it is often possible to conceive of the linguistic objects in such a way as to eliminate potential sources of ambiguity and additional external mechanisms.</Paragraph>
    <Paragraph position="1"> To illustrate our approach we will propose an account of a subset of Verb Alternation phenomena which rely on what are essentially underspecilled lexicM entries. The lexicon will then contain one (verbal) entry and the system will rely only on the existing resources (the type hierarchy)  to provide the different interpretations of the predicate which license the distinct eoml)lementation patterns. Analysis is incremental and deterministic and the procedure relies mainly on what we will call 'trivial type inference'. In the sections that follow \[irst we discuss the linguistic at)l)roaeh underlying our proposal, second we eOmlmre our proposal to existing underspecification tq)proaches and finally, we give some details of the implementation which relies on no special fc'+~tures or external devices.</Paragraph>
    <Section position="1" start_page="164" end_page="166" type="sub_section">
      <SectionTitle>
2.1 -Underspeeificatlon
</SectionTitle>
      <Paragraph position="0"> We will exemplify ore- approach by treating a subset of verb alternati(ms which conform to the following general schema (\]). These include the so-called spray/load (locative) Mtornation, the wipe~clear a.lternation, the b~vak/hit alternation etc (l,evin 19!)a).</Paragraph>
      <Paragraph position="1"> (~) v Nl'j \[l'~ Np~\] -+ v NP~ \[P~ Nt'.j\] We adopt the view that verb l)redicatcs are open to contextual information (which ntttst I)e contrasted to the approaches whereby verb predicates are treated am fully formed objects which dictate tit+; exact nature of their dependents). Consider the  predicate load: (2) The peasant loaded the horses.</Paragraph>
      <Paragraph position="2"> (3) The peasant loaded the horses on the boat. (4) The peasant loaded tit(: horses with hay. (2) is ambiguous I)ctwc,m (3)and (4:) ,inch one  of which is not ambiguous. 'l'he contextual factor that resolves the ambiguity is the semantics of the head of the prepositional complement which here is tt~ken to specii~y whether the direct ob.ieet of the verb is understood as the location and the oblique complement as the locatum or v\[ee versa. The crutim a.ssumption here is that prepositions have their own semantics, an idea first exploited in ((lawron 1!)86).</Paragraph>
      <Paragraph position="3"> We use/IPSG to model our approach. (5) gives the fragment of the type system constraining the wdues of the SYNSEMILOC\]CONTINUCLEUS path in the (word) description of prepositions which participate to the locative alternation l)henomenon.</Paragraph>
      <Paragraph position="5"> We %rthermore assume that the semantics of the predicates include a pointer to the semantics of the prepositional complements they license. This pointer is included as an extra feature of the value of SYNSF, IVI\[LOC\[CONT\[NUCLEUS. This feat/ire we ttalile SFM(ANTIC) CONS('FII.AINq'S) a, tl(\[ we make it apl)rol)riate tbr the same values that the prepositional SYNSEMII,OC\[CONT\]NUCI,EUS is assigned. The lexica\] entry for to load would look as in ((~).</Paragraph>
      <Paragraph position="7"> The lexieal entry for the pre4)osition with is  given I)el()w: ......... (+:~) + ..... ....... + .......... ,::::, with_contact Alt{+2 ~\[~\] J (6) is an mtdcrspcciJicd entry which gets filrther specified at parsing time when art appropriate PP is a tta+ehed, l&amp;quot;or instance, if a with-\[)l ~ is encountered, then tm interpretation according Lo which the location sm'faccs as the direct object of the verb is injured.</Paragraph>
      <Paragraph position="8">  There are some theoretical reasons why we have chosen to include a &amp;quot;pointer&amp;quot; to prepositional semantics rather than making it compatible (uui\[i-M)le) to verbal semantics as Weehsler (Wechsler 1994) has proposed, l,'irstly, if verl)al and prepositional semantics were uniliable then we would not bc able to explicitly state, in the semantics the relation which each feature structure encodes as there would be a clash of constants (relation nanms are constants). Secondly, identifying the semantics of verbs with ttmt of prepositions does not allow for expressing certain types of diverse behaviour within the class of a.lternating verbs. For instauce, both load and stuff show locative, alternation, but only the former admits optional PP complements. With to stuff the interpretation under which leeslion is a direct object admits an optional P I ) eotnplement (8) while the interpretation under u, hich Iocatum is a direct argument admits an obligatory one (9). Similarly, while both versions of to load are related to passive adjectives (loaded carl, loaded hay), only the &amp;quot;location&amp;quot; version is related  to such an adjective in the case of to stuff (stuffed pillow, *stuffed feathers). The exact treatment of such phenomena, however, goes beyond the scope of our discussion here which concentrates on the use of underspecification.</Paragraph>
      <Paragraph position="9">  (8) Mary stuffed the pillow with feathers.</Paragraph>
      <Paragraph position="10"> (9) Mary stuffed the feathers into the pillow.</Paragraph>
      <Paragraph position="11"> Optionality of PP complements can also be captured easily with this proposal. With to load (2), (3) and (4), the PP complement is optional. The grammar must have access to three different ver- null sions of to load, one with zero PP complements and two with a PP complement participating in the alternation discussed above. One approach would involve defining two lexieal rules; an alternative would be to express all three possibilities directly. Both are problematic, of course. Consider the situation when the grammar has two PS rules for VPs, one for discharging a \[NP,NP\] SUBCAT list and one for discharging a \[NP,NP, PP\] list. Without harming generality, assume that the bivalent version of to load is in the lexicon and two lexical rules generate the trivalent versions. To process a trivalent version, the parser will backtrack on the bivalent version, will use a lexical rule and then, it will either succeed or it will backtrack again and use a second lexical rule.</Paragraph>
      <Paragraph position="12"> To avoid this, the following solution may be adopted. First, the type system is augmented to allow for declaring the property of being an optional or an obligatory prepositional complement, as in figure 1.</Paragraph>
      <Paragraph position="13"> Second, a PS structure rule is introduced of the following sort:</Paragraph>
      <Paragraph position="15"> Only one trivalent, underspecified version of to load is necessary. The parsing of a trivalent version as before would involve backtracking on the rule dealing with optional complements but then the rule dealing with obligatory ones would be chosen and it would succeed anyway.</Paragraph>
      <Paragraph position="16"> Only limited inference power is necessary for this set up to work: the system must be able to infer that the unification of a subtype with its supertype is of the type of the subtype. This 'trivial inference power' is independently needed to deal, for instance, with (10): if NP1 is a subtype of NP then rule (10) will work only if trivial inference power is available when the sequence NP1, VP is</Paragraph>
      <Paragraph position="18"> There are proposals in the literature which build on the idea of using underspecified entries.</Paragraph>
      <Paragraph position="19"> However, several of them use additional, external powerful mechanisms to simulate type inference.</Paragraph>
      <Paragraph position="20"> (van Noord ~: Bouma 1994) use underspecified verb entries and FP~OLOG delayed evaluation techniques to insert adjuncts in Dutch VPs without using lexical rules which would cater for the necessary variations of the subcategorisation list of verbs. In another proposal using underspeciflcation (Sanfilippo 1995) type inference (feature structure grounding) is simulated by relying on an external mechanism as powerful as PItOLOG.</Paragraph>
      <Paragraph position="21"> In dealing with different complementation pattern phenomena, Sanfilippo constructs type system fragments where the meet of the alternative complements is defined and subtypes verbs according to complement types. Therefore, the information about the alternation is duplicated in the type system as it is encoded both on tile complement types and the verb types. The same information is encoded again on a table of clauses which relate a verbal &amp;quot;meet&amp;quot; type with a maximal complement type and a maximal verb type. Such type resolving clauses are provided for each alternation pattern.</Paragraph>
      <Paragraph position="22"> PS rules are annotated with procedures which pick up the correct verb type resolving clause when the appropriate complement is encountered. Both the clauses and the searching procedures are mechanisms external to the inferencing mechanism that is directly related with the type system. Sanfilippo's approach, though powerful and flexible, seems extravagant for phenomena like verb alternations of the kind discussed here as well as inflection phemomena of the kind discussed in (Krieger &amp; Nerbonne 1993). In such cases the system can take advantage of the fact that type inference can be driven by the combination of the information that is related to two separate strings (preposition and verb, verb ending and verb stem) as is exemplified in our proposal.</Paragraph>
      <Paragraph position="23"> Furthermore, in our approach no horizontal relations exist as the lexicon contains only one entry and no other entry is ever generated. Instead, the single lexical entry is interpreted on the fly, each time according to well-specified constraints. Consequently, no ambiguity problems result with a nice effect, on parsing time. In this sense, using underspecification defined in the type system is more  econolnic than using lexical rules or a &amp;quot;static&amp;quot; version of underspecification which is defined in the lexicon. For instance, (Krieger &amp;: Nerbonne :1993) have used a specialised macro, the so-called distributive (or named) disjunction, in a treatment of German verb inflectional morphology: While it is true that distributive disjunction does not add any expressive power to the system (though a piece of machinery, the specialised macro, must be supported), if the macro is ever called all the legal combinations are thereby gem crated and added to the lexicon. In this, the situation is precisely the same us with lexieal rules, for in each case, what is provided is simply a compact representation of an ambiguity.</Paragraph>
      <Paragraph position="24"> This can be also exemplified from the domain  of Verb Alternation phenomena. (11) will generate two lexical entries with at1 identical I'IION string. (\]*) .... &lt; ....... &gt; \[ ...... &lt;N\[&amp;quot;,\[~ , WI'I'H_I\]IJ:\[~&gt; \]V ....... ILo&lt;: \[ ...... &lt;Ivp:lTi\] .... pp:~&gt; IIIlll, load .m( .......</Paragraph>
      <Paragraph position="25">  Unlike lexical rules, ou\]: approach does }lot face any blocking problem. A w:rbal predicate that does not alternate (such us the predicate to put (12),(13)), is assigued the upppropriate most specific semantics for its SYNSEM ILOC iCONTiNUOLI~US iSEM .(iONS attribute ---for to put that would be on-contact in order to make sure that the locatum argument always surfaces as the direct object; of the verb predicate.  (12) John put his shoes on the shelf.</Paragraph>
      <Paragraph position="26"> (13) *John put the shelf with his shoes.</Paragraph>
    </Section>
  </Section>
  <Section position="3" start_page="166" end_page="166" type="metho">
    <SectionTitle>
3 Implementation
</SectionTitle>
    <Paragraph position="0"> The appro~tch described in Section 2 can bc i\[nplemented in any environment that supports Typed Inheritmme because it is monotonic and demands only 'trivial inference power'. For the purposes of experimentation a grammar fragment was iml)lemented in the ALEI' system - a lean formalism with a simple inheritance type system, and a siml)\]e context free rule backbone. I:'rocessiug in this system is normally divided into separate structure building and feature decoration rule coati&gt;orients , however for our l&gt;ui'poses no use was made of this distinction.</Paragraph>
    <Paragraph position="1"> For the construction of the VP, a simple rule</Paragraph>
    <Paragraph position="3"> The relewmt lexical entries for the fragnient were as follows. The verbal entry (load) subcategorizes for a single NP subject and NI' and PP complenmnts (15). This entry has underspeeified semantics with respect to the semantic constraints on its second and third argmnents (as suggested in (8)). These are provided by (structure sharing with) the SI,'M_CONSTI~ feature of third argument, the prepositional phrase (the variable 'Arg3').</Paragraph>
  </Section>
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