Lexical Rules: What are they? Andrew Bredenkamp, Stella Markantonatou and Louisa Sadler Department of Language and Linguistics University of Essex, UK {andrewb,m~trks,louisa}~essex.~c.uk Abstract ive Alternation, ctc) and word formation phenom- ena (inflectional and derivational morphology). In Horizontal redundancy is inherent to lex- fact, Pollard and Sag also refer to declension class ica consisting of descriptions of fnlly membership and similar facts as horizontM rela- formed objects. This causes an unwel- tions, and as we shall see, the boundary between come expansion of the lexical database vertical and horizontal relations is not immutably and increases parsing time. To eliminate fixed once and for all. it, direct relations between descriptions The notion of lexical rule is often given some of fnlly formed objects are often defined. status at the level of linguistic or psychological These are additional to the (Typed Mul- theory. (Pollard & Sag 1987) make reference to tiple) Inheritance Network which already a generative or procedural interpretation of lexicM structures the lexicon. Many implement- rules as a deductive mechanism which can be de- ations of horizontal relations, however, ployed on a needs only basis, for example, to gen- fail to generate lexieal entries on a needs- erate words from a single base form. The concep- driven basis, so eliminate neither the tion of lexical rules as essential generative devices problem of lexicon expansion nor that of (rather than static statements expressing (sub-) inefficient parsing. Alternatively, we pro- regularities), is shared in much in:llnential work pose that lexical entries are descriptions (e.g. (Bresnan 1982), (Pinker 1989)), although of objects open to contextnal specifica- it is by no means universM, even within tIPSG. tion of their properties on the basis of Viewed from an implcmentational perspective, on- constraints defined within the type sys- the-fly application of lexicM rules brings with it a tem. This guarantees that only those number of distinct advantages which follow from grammatical lexical entries are infered the drastic reduction in the size of the lexicM data- that are needed for efficient parsing. The base (lexical construction is less time consmning proposal is extremely modest, making and parsing time should be reduced as lexical look use of only basic inference power and ex- up is less ambiguous, etc). At first sight then it ap- pressivity. pears that the benefit of adding an external Lexical l(ule component outweighs the disadvantages (ex- ternal powerNl mechanisms). We will first show 1 Lexical Rules: what are they? that their role is less clear than this suggests and Within the strongly lexical framework of HPSG, certainly more problematic, before suggesting in lexi('al rules are used to express relations among Section 2 an alternative which eschews any extra descriptions ..... a kind of indirect "horizontal re- mechanisms. latedness" (Pollard & Sag 1987, 209) which can be 1.1 Horizontal and Vertical Redundancy contrasted with the vertical relations between the type(s) of lexical elements. Type relations are, of The parallel drawn above between vertical related- course, captured directly as the monotonic (typed) ness (expressed with the type system) and ho- multiple inheritance network itself, which struc- rizontM relatedness among descriptions of fully tures the lexicon. formed objects is however rather misleading. Typical examples of horizontal redundancy in Monotonic multiple inheritance networks are most the hierarchical lexicon thus conceived arc the Al- naturally used to represent generalisations over ternation phenomena (e.g. Dative Shift, the Locat- the properties that (groups of) linguistic objects 163 share - inspection of any network will eonfirin constrained to account for 'exceptional' behaviour, that they are usually deployed to express what that is, for those words which do not participate is essentially a componential analysis of objects to a given horizontal relation despite the fact that and of the relationship between them (defined on their description makes them appropriate candid- the basis of this analysis). On the other hand, ares for the relation (verb alternations offer several horizontal relations among descriptions (very of- examples of these situtation, for instance, 'giving' ten lnodelled by means of lexical rules) are es- verbs which do not exhibit the so-called 'dative sentially relations holding directly between objects shift' phenomenon). themselves. While this intuition is clear, this is Modelling of'exceptional' behaviour leads either much less adequate an approach for morpholo- to an extreme complexity of the type system or gical relatedness, where a componential approach to non-lnonotonic solutions (Flickinger 1987) b(> may often appear just as natural as an object re- cause it turns out that certain horizontM relations, latedness view, especially if the formalism includes usually defined over types, nmst be blocked for in- fimctionally dependent values, permitting the ex- dividuM objects. pression of allomorphic wtriation and the like. In fact, many putatively horizontal relations may be 1.3 hnplementing Horizontal Relations simply re-expressed within a type hierarchy by Several different implementations of horizontal re- viewing them Dom a compouential perspective, lations exist. All of them add extra machinery obviating the need for expressing them on the %o- and some add extra expressive power to the core rizontal" dimension which may lead 6o the use of mechanism. lexical rules. 13ut this is only possible once one Most frequently, horizontM relations are imple- frees oneself from a view of lexical relatedness as mented as unary rules operating at parsing time something which holds essentially between words within a dcrivationM component. Such a compon- (objects which correspond to maximal types, that ent is tided to the inheritance machinery fbr in- is types at the bottom of the type hierarchy). dependent reasons, mainly because of' the limited llorizontal relations are perhaps most naturally expressivity of the type systeul. With Ll/.s, some captm:ed by an extra device (Ll{s) external to the lexical entry is considered as 'basic' and all other lexical network and associated inference mechan- lexical entries are derived fl:om it introducing oth- ism-- see (Krieger gc Nerbonne 1993) and (Cal- erwise unjustified directionality to the grammar. cagno 1995) for recent ItPSG proposals. Some re- In addition, the derivational implementation of ho- cent work ((Mem'ers 1995) and (l{iehemann 1994:)) rizontal relations fails to produce lexical entries a.s partly departs fl:orn this view by expressing rela- needed, instead, it produces lexica.l entries accord- tions between objects using the vertical axis (that ing to the system's internM algorithm of searching is, using the type system), but again tile starting the rule space. Considerable ambiguity is intro- point is 'complete' lexical objects. duced with unpleasant results for parsing time. 1.2 Why avoid Horizontal RelationsY Extra machinery for blocking these rules in or- der to account for exceptional behaviour is also Horizontal relations have a number of undesirable necessary. features as well as requiring an external meclmn- Alternatively, Lt{s may be compiled out bat, ism. HorizontM relations (between objects) are nnder this approach too, problems like direction- in principle pretty much unconstrained. Vertical ality and the blocking of Ll{s as well as expensive relations are more constrained becmlse they are ambiguity at parsing time remain unsolved. based on componential analysis, starting out Dora the set of properties that objects have. On the 2 An alternative proposal other hand, any object can be related to any other object by stipulation in an external mechanism. In In this paper we explore an alternative to hori- architectural terms, it is simply accidental (if for- zontal relatedness which exploits the idea that it is tuitous) that lexical rules are often used to relate often possible to conceive of the linguistic objects minimMly different objects -- they are capable of in such a way as to eliminate potential sources much more promiscious behaviour. of ambiguity and additional external mechanisms. This state of,affairs is amply demonstrated in To illustrate our approach we will propose an ac- the literature, which abounds with attempts to count of a subset of Verb Alternation phenom- constrain horizontal relations by appeal to subsi- ena which rely on what are essentially underspe- diary principles (predicate locality in LFG, con- cilled lexicM entries. The lexicon will then con- straints of a psycholinguistic nature in the work of tain one (verbal) entry and the system will rely (Pinker 1989), etc). Horizontal relations must be only on the existing resources (the type hierarchy) 164 to provide the different interpretations of the pre- pointer is included as an extra feature of the dicate which license the distinct eoml)lementation value of SYNSF,IVI[LOC[CONT[NUCLEUS. This fea- patterns. Analysis is incremental and determin- t/ire we ttalile SFM(ANTIC) CONS('FII.AINq'S) a,tl([ istic and the procedure relies mainly on what we we make it apl)rol)riate tbr the same values that will call 'trivial type inference'. In the sections the prepositional SYNSEMII,OC[CONT]NUCI,EUS is that follow [irst we discuss the linguistic at)l)roaeh assigned. The lexica] entry for to load would look underlying our proposal, second we eOmlmre our as in ((~). proposal