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<Paper uid="C00-1003">
  <Title>Selectional Restrictions in HPSG</Title>
  <Section position="4" start_page="15" end_page="15" type="metho">
    <SectionTitle>
2 Background restrictions
</SectionTitle>
    <Paragraph position="0"> The first way to accommodate selectional restrictions in HPSG USeS the CONTEXTIBACKGROUND (abbreviated here as CXlBG) feature, which Pollard and Sag (Pollard and Sag, 1994) reserve tbr &amp;quot;Micity conditions on the utterance context&amp;quot;, &amp;quot;presuppositions or conventional iml)licatures&amp;quot;, and &amp;quot;at)prot)riateness conditions&amp;quot; (op cit pp. 27, 332). To express selectional restrictions, we add qfpsoas (quantifier-flee parameterised states of atfairs) with a single semantic role (slot) iI1 CXIBG. 1 For exanlple, apart fi'om the eat qf'psoa in its NUCLEUS (NUt), the lexical sign tbr &amp;quot;ate&amp;quot; (shown in (10)) would introduce an edible qfpsoa in Bo, requiring \[\] (the entity denoted by tile object of &amp;quot;ate&amp;quot;) to be edible.  In the case of lexical signs for proper names (e.g. (11)), the treatment of Pollard and Sag inserts a naming (namg) qfpsoa in BG, which requires the BEARER (BRER) to by identifiable in the context by means of the proper name.</Paragraph>
    <Paragraph position="1">  (11) also requires the bearer to be a man.</Paragraph>
    <Paragraph position="2"> (11) &amp;quot;PIION &lt;ToTrt&gt; oa,l, \[,,,:AD</Paragraph>
  </Section>
  <Section position="5" start_page="15" end_page="17" type="metho">
    <SectionTitle>
CONT \[INDEX \[RESTR ~}\]
</SectionTitle>
    <Paragraph position="0"> Tile ItPSO t)rinciples that control the propagation of the BG feature are not fully developed.</Paragraph>
    <Paragraph position="1"> For our purposes, however, tile simplistic princo)Ic of contextual consistency of Pollard and Sag will suffice. This principle causes the BG value of each phrase to be the union of tile BG values of its daughters. Assuming that the lexical sign of &amp;quot;keyboard&amp;quot; is (12), (10)-(12) cause  (1) to receive (13), that requires \[\] to denote an edible keyboard.</Paragraph>
    <Paragraph position="2"> 1To save space, we use qfpsoas wherever Pollard and  Sag use quantified psoas. We also ignore tense and aspect here. Consult (Androutsopoulos et al., 1998) for the treatment of tense and aspect in our ltpsG-based database interface.</Paragraph>
    <Paragraph position="4"> A(:('ording t,, (13), to accept (1), one has to place it; ill ;~ st)ecial conte.xt where edible keybonrds exist (e.g. (1) is thlMtous if it reM's to ;~ miniature cho(:ol~te keyt)oard). Su(:h (:ontexts, however, are rare, ;rod hen(:e (1) sounds generally odd. Alternatively, one has to relax the B(~ constraint that the keyboard 1111181; BC edi|)le.</Paragraph>
    <Paragraph position="5"> We assmne that special contexts ~dlow t)a.rticu l~r BG constraints to be relaxed (this is how we wouht a(:(:omlt fin&amp;quot; the use of (1) ill ~L circus context), \])ut we (Io not ll;we any t'ornl~d lne(:hanisnl to sl)e(:itly exactly when B(~ (:(mstr;dnts ('~m l)e relnxcd.</Paragraph>
    <Paragraph position="6"> Similnr connnents apply to (3). Assuming th;Lt the sign of &amp;quot;req)aired&amp;quot; is (ld), nnd that the sign of &amp;quot;teclmi(:iml&amp;quot; is similar to (12) except that it; introdu('es a technician index, (3) receives a sign theft requires the repairer to 1)e a technician who is all artifact. U k~(:hnicians, however, are generally not artifacts, which accounts for the oddity of (3).</Paragraph>
    <Paragraph position="7">  WOilkl the system fig.re out fl'om (13) that (:1) is pragm~tieally odd? Among other things, it wouhl need to know that keyl)o~r(ts ~rc not edible. Similarly, in (2) it would need to know that |)~m~m~s are edible, ~md in (3) (d) it; would need I;() 1)e nwarc that technicians are. llot artifacts, while keyboards m:e. Systenls that employ selc(:tionnl restri(:tions usunlly encode knowledge of this kind in the. fol:nl of sort hierarchies of worhl entii;ies. A siml)listic exmnt)le of such n hierm:chy is det)i('ted ill tigure 1. The hierarchy of tigure \] shows thnt nil lllell &amp;lid technicians are 1)ersons, all 1)ersons are ~tniln;~|;e entities, all aninlate entities are t)\]lysi(:al ol)je(:ts, mitt so on. Some (1)ut not all) persons are 1)oth teehni(:ians :rod lnen at the same time; these t)ersons are nmml)ers of I;he male_tech sort. Similarly, all l)mlmms are edil)h; ;rod liot artifacts. No person is e(lil)le, because the sorts person and edible h~we no (:onnnon su|)sorts.</Paragraph>
    <Paragraph position="8"> It is, of course, extremely difficult to constru('t hierm'chies th~Lt include all the sorts of world entities. Ill natural bmguage systenls that target sl)ecifi(: and restricted dolmfins, however, constructing such hier;~rchies is feasible, because the relevant entity sorts and the possible hierarchical reb~tions between them are limited.</Paragraph>
    <Paragraph position="9"> In naturM lmlguage database interfimes, tbr example, the relevant entity sorts and the relations between theln nre often identilied during the, (tesing of the database, in the tbrm of entity-relatiolMli 1) diagrams. We also note l;h;~t large-scah; smmmtic sort hierarchies are already ill use ill artiticinl intelligence ~md natural  language gener~tion projects (tbr example, Cyc (Lenat, 1995) and KPML'S Upper Model (Bateman, 1997)), and that the techniques that we discuss in this paper are in principle compatible with these hierarchies.</Paragraph>
    <Paragraph position="10"> To decide whether or not a sentence violates any selectional restrictions, we collect from the CONT and BO features of its sign ((13) in the case of (1)) all the single-role qfpsoas for which there is a sort in the hierarchy with the same name. (This rules out single-slot qt~)soas introduced by the CONTs of intransitive verbs.) The decision can then be seen as a constraintsatisthction problem, with the collected qfpsoas acting as constraints. (15) shows the constraints tbr (1), rewritten in a tbrm closer to predicate logic. HPSG indices (the boxed nmnbers) are used as variables.</Paragraph>
    <Paragraph position="12"> Given two contstraints cl, c2 on the same varial)le, c~ subsumes c2 if the corresponding hierarchy sort of cl is an ancestor of that of c2 or if cl = c2. c~ and c2 can be replaced by a new single constraint c, if cl and c2 subsume c, and there is no other constraint d which is subsumed by cl,c2 and subsumes c. c and c' must be constraints on the same variable as ct, c2, and must each correspond to a sort of the hierarchy. If the constraints of a sentence can be turned in this way into a tbrm where there is only one constraint fbr each variable, then (and only then) the sentence violates no selectional restrictions.</Paragraph>
    <Paragraph position="13"> (15), and cdil, ,'am ot be rep\]aced by a single constraint, because keybd and edible have no common subsorts. Hence, a selectional restriction is violated, which accounts for the oddity of (1). In contrast, in (2) the constraints would be as in (16).</Paragraph>
    <Paragraph position="14">  (16) banana(~) A man(m) A edible(~\]) banana(~\]) and cdible(F~) can now be replaced by banana(F~), because both subsume banana(~\]), and no other constraint subsmned by both banana(~\]) and cdible(~) subsulnes banana(~). This leads to (17) and the conch&gt; sion that (2) does not violate aw selectional restrictions.</Paragraph>
    <Paragraph position="15"> (17) banana(E\]) A man(D\])  This constraint-satisfaction reasoning, however, requires a set)arate inferencing component that would be pipe-lined after the parser to rule out signs corresponding to sentences (or readings) that violate selectional restrictions. In the next section, we discuss an alternative approach that allows hierarchies of world entities to be represented using the existing HPSG framework, and to be exploited during parsing without an additional inferencing component.</Paragraph>
  </Section>
  <Section position="6" start_page="17" end_page="18" type="metho">
    <SectionTitle>
3 Index subsorts
</SectionTitle>
    <Paragraph position="0"> HPSG has already a hierarchy of feature structure sorts (Pollard and Sag, 1994). This hierarchy can be augmented to include a new part that encodes intbrmation about the types of entities that exist in the world. This can be achieved by partitioning the ref HPSO sort (currently, a leaf node of the hierarchy of feature structures that contains all indices that refer to world entities) into subsorts that correst)ond to entity types. To encode the information of figure 1, rEf would have the snbsorts abstract and physical, physical would have the subsorts animate, edible, inanimate, and so on. That is, referential indices are partitioned into sorts, mid the indices of each sort can only be anchored to world entities of the corresponding type (e.g. keybd indices can only be anchored to keyboards).</Paragraph>
    <Paragraph position="1"> With tiffs arrangement, the lexical sign for &amp;quot;ate&amp;quot; becomes (18). The Bo edible restriction of (10) has been replaced by the restriction that the index of the object must be of sort edible.</Paragraph>
    <Paragraph position="3"> Similarly, the sign for &amp;quot;Tom&amp;quot; becomes (19) (cE (11)), and the sign for &amp;quot;keyboard&amp;quot; introduces an i,dex of sort k vbd as shown in (9O) (cf. (12)).</Paragraph>
    <Paragraph position="5"> Unification of indices pro(:eeds in the, s;lille maturer as unificatioll of all other typed feature structm:es ((Jarlienter , 1!)!/2). 'Fhe parsing of (\]) iIOW fails, 1)ecause it al, te, nq)ts to unilly an il dox or (i,lt,:o,hl(::ed t/y with an index of so,*; t,:eybd (introduced t,y (20)), and no Ill'SO sorl; is sul)sumed l)y both. in (:ontrast, the parsing o17 (2) would su(:('eed, because the sign of &amp;quot;bmuma&amp;quot; would introduce an index of sort banana, which is a sut)sort of edible (Iigur(~ 1); hence the two indi(:es can 1)(', ratified. (3) and (4) would l)e l)ro('essed sinfilarly.</Paragraph>
    <Paragraph position="6"> in (7) and (8), there would 1)e two lcxi(:nl signs for &amp;quot;ln'illtcr&amp;quot;: one inl;ro(lu('ing ml index of sort pri'nter_pe'r.s'o'n, and one im:o(lu(:ing an index of sort pri'nte'r_periph,(~'ral. (printe'r4)er.~'on and l)rinter_periph, cral would t)e daughters of person and art'@tel respectively in tigure 1.) The sign for &amp;quot;repairc, d&amp;quot;, would require the index of its ol)je(:t to be of sort arl,'l\[fact, and l;he sign of &amp;quot;(:ail(~d&amp;quot; wou\](l re(tuire its sul)je('l; index to t)e of sort per,so'n. This (:orre(:tly admits only the reading where the rel)aire(l entity is a (:Omlml;er peripheral, ml(t l;tm (:aller is ;t t)(',rson. Simil~tr llleC\]iallisnls (;;/,ll })e llse(t to (l(~,\[;(!lTillille t;tlP, (;of reel; reading of (9).</Paragraph>
    <Paragraph position="7"> With the al)proa(:h of this see, lion, it; is also possible to speciily seh;ctional restrictions in the declarations of qflIsoas in the Ill'SO hierarchy of feature structures, as shown in tigure 2, rather than in the lexi(:on. 2 When the same qft)soa is used in several lexical signs, this saves having to repeat tile same, selectional restrictions in each one of the lexical signs. For example, the verbs &amp;quot;rq)air&amp;quot; and &amp;quot;Iix&amp;quot; iiiay both introduce a repair qfpsoa. The restriction that the repaired entity must be an artifact can lie sl)eeified once in the declaration of repair in the hierarchy of feature structures, rather than twice in the lexieal signs of ~'cl)air&amp;quot; and &amp;quot;fix&amp;quot;.</Paragraph>
    <Paragraph position="8"> 2Additional layers can be included betwc,(m qfpsoa and the leaf sort;s, as sketched in section 8.5 of (Pollard and Sag, 1994), to group together qfpsoas with common selnalltifi roles.</Paragraph>
  </Section>
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