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<Paper uid="P91-1028">
  <Title>Multiple Default Inheritance in a Unification-Based</Title>
  <Section position="4" start_page="217" end_page="219" type="metho">
    <SectionTitle>
3 Example Analyses
</SectionTitle>
    <Paragraph position="0"/>
    <Section position="1" start_page="217" end_page="218" type="sub_section">
      <SectionTitle>
3.1 German Separable Verbs
</SectionTitle>
      <Paragraph position="0"> Two large classes of German verbs are the separable and inseparable prefixed compound verbs.</Paragraph>
      <Paragraph position="1"> The former are of interest syntactically because, as their name suggests, the prefix is a bound SSee 3.2 below for a case where such multiple matches are desirable.</Paragraph>
      <Paragraph position="2"> morpheme only in certain syntactic environments, namely when the verb is untensed or head of a verb-final clause. 9 Members of both classes share morphological, but not necessarily syntactic, properties of the verb which corresponds in form to their stem. The separable-prefix verb weglau/en ('run away') and inseparable verlau/en ('elapse') are two such verbs, which the lexicon should be able to relate to their apparent stem lau/en ('run'). Since word definitions are classes, they can be inherited from like any non-lexical class. Thus the lexical classes verlaufen and weglaufen may inherit from lanfen, itself a lexical class: xdeg</Paragraph>
      <Paragraph position="4"> 9Within the syntactic analysis assumed here, the distribution of verbs is controlled by a binary feature inv, whose value in these contexts is no.</Paragraph>
      <Paragraph position="5"> lea number of simplifications have been made here; \]aufen is in reality a member of a subclass of the strong verbs, and the verb class itself has been truncated, so that it accounts for only bare infinitive and first person singular present tense indicative forms. Past participle formation also interacts with the presence of separable and inseparable prefixes.</Paragraph>
      <Paragraph position="6">  The lexical classes weglaufen and verlaufen each have two immediate superclasses, containing information connected with the prefix and stem.</Paragraph>
      <Paragraph position="7"> The classes weg and vet set the value of the morph:prefix path of the verb (overriding the value given in the main set of verb), and specify inheritance from the separable and non.separable classes respectively. The former of these unifies the variable Prefix with either the empty string (in the case of tensed 'inverted' verbs) or the value of morph : prefix (for other variants), while the latter sets the value uniquely for all forms of the verb in question. As the value of sere is fixed in the main equation set ofweglaufen and verlaufen, the corresponding equation in laufen is overridden, but Base.stem unifies with lauf. Finally, in verb, the main set supplies default values for Prefix and morph : prefix (which in the cases under consideration will not be applicable), unifies P_bs with the result of concatenating the strings Prefix and Base_stem, and for each value of syn infl assigns to form the concatenation of P_bs with the appropriate sufftx string.</Paragraph>
      <Paragraph position="8"> Values for sere (antics) are provided in main set equations; those in weglaufen and verlaufen are thus correctly able to override that in laufen.</Paragraph>
    </Section>
    <Section position="2" start_page="218" end_page="219" type="sub_section">
      <SectionTitle>
3.2 English Irregular Verbs
</SectionTitle>
      <Paragraph position="0"> In most cases, lexical items that realize certain morphosyntactic properties in irregular forms do not also have regular realizations of those properties; thus *sinked is not a well-formed alternative to sank or sunk, on the analogy of e.g. walked.</Paragraph>
      <Paragraph position="1"> This phenomenon has frequently been discussed in both theoretical and computational morphology, under the title of 'blocking', and it appears to provide clear motivation for a default-based hierarchical approach to lexical organization. 11 There are exceptions to this general rule, however, and inheritance mechanisms must be sufficiently flexible to permit deviation from the strict behaviour illustrated above.</Paragraph>
      <Paragraph position="2"> Consider the small class of English verbs including dream, lean, learn and burn; these have, for many speakers, alternate past finite and past participle forms: e.g. dreamed and dreamt. The following fragment produces the correct pairings of strings and feature structures, the written form of the word being encoded as the value of the form llSee e.g. Calder (1989).</Paragraph>
      <Paragraph position="3"> feature: 12</Paragraph>
      <Paragraph position="5"> The main set equations in s/nk override those in its superclass verb, so that the variants in the latter class which give rise to past participle and past tensed forms associate the appropriate information with the strings sunk and sank, respectively. The class walk, on the other hand, contains nothing to pre-empt the equations in verb, and so its past forms are constructed from its value for base and the suffix string ed.</Paragraph>
      <Paragraph position="6"> The lex/cai class dream differs from these in haying as one of its direct superclasses dual-past, which contains two variant sets, the second of which is empty (recall that variant sets are preceded by the vertical bar 'I'). Moreover, this class is more specific than the other superclass verb, and so its equations assigning to PaP_Form and P_Fin_Form the string formed by concatenating the value of base and t have precedence over the contradictory statements in the main set of verb.</Paragraph>
      <Paragraph position="7"> Note that this set also includes a disjunctive constraint to the effect that the value of morph in this FS must be either pastfinite or pastnonfinite.</Paragraph>
      <Paragraph position="8"> The dual_past class thus describes two feature IZAgain, the analysis sketched here is simplified; several variants within the verb class have been omitted, and all infleetional information is embodied as the value of the single feature morph.</Paragraph>
      <Paragraph position="9">  structures, but adds no information to the second. The absence of contradictory specifications permits the equations in the main set of verb to apply, in addition to those in the first variant set of dual-past. The second, empty, variant set in dual-past permits this class also to inherit all the properties of its superclass, i.e. those of regular verbs like walk; among these is that of forming the two past forms by suffixing ed to the stem, which produces the regular dreamed past forms.</Paragraph>
    </Section>
    <Section position="3" start_page="219" end_page="219" type="sub_section">
      <SectionTitle>
3.3 Word-Form Manipulation
</SectionTitle>
      <Paragraph position="0"> The string concatenation operator '&amp;&amp;' allows the lexicon writer to manipulate word forms with ELU equations and macros. In particular, &amp;t can be used to add or remove prefixes and suE3xes, and also to effect internal modifications, such as German Umlaut, by removing a string of characters from one end, changing the remainder, and then replacing the first string. In this section we show briefly how unification, string concatenation, and defensible inheritance combine to permit the analysis of some of the numerous orthographic changes that accompany English inflectional sufftxation.</Paragraph>
      <Paragraph position="1"> The inflectional paradigms of English nouns, verbs, and adjectives are complicated by a number of orthographic effects; big, hop, etc. undergo a doubling of the final stem character in e.g. bigger, hopped, stems such as/oz, bush, and arch take an epenthetic * before the plural or third singular present suiflx s, stem-final ie becomes y before the present participle suifL~ ing, and so on. Peripheral alternations of this kind are accomplished quite straightforwardly by macros like those in the following lexicon fragment (in which invocations of user-defined macros are introduced by ': ,):is  13As before, this is s somewhat sbbre~sted version of s full descrip~on; the verb and vo~bJpolliag classes require additional variant sets to account for other morphosyntsc~c prope~|es. Other st~ng-predicste macros, in particular OK, must be defined in order to ester for the ~ range of spelling</Paragraph>
      <Paragraph position="3"> Two macros definitions are shown here; Final_Y= is true of a pair of strings String and Prefix iff String consists of Prefix followed by y and the final character of Prefix is one of the set denoted by the disjunction b/c.., z, while Final_Sibilant is true of a given string iff that string terminates in sh, ch, s, z, or z. OK is a macro which is true of only those strings to which neither Final.Sibilant nor Final_Y apply.</Paragraph>
      <Paragraph position="4"> The class verbJpellJ.ng contains three variant equation sets, the first two of which assign values to variables according to the form of the string which is the value of the base feature. If Final_Y= is applicable, Base.P-PSP is unified with the concatenation of the second argument to the macro (e.g. tr) and is, while Base_3SG is unified with e.g. tr and i.</Paragraph>
      <Paragraph position="5"> If FJ.na1.Slbilant is applicable, then Base.3SG is unified with the concatenation of the value of base (e.g. watch) and e. If neither of these is applicable (because the base string does not match the patterns in the macro definitions), the variables are bound not within this class, but in the main equation set of its superc\]ass verb. Here, their values are unified directly with that of base, and the eventual values of the form feature result from concatenation of the appropriate suiflx strings, giving values of watched, watches, tried, and tries.</Paragraph>
    </Section>
  </Section>
  <Section position="5" start_page="219" end_page="220" type="metho">
    <SectionTitle>
4 Summary
</SectionTitle>
    <Paragraph position="0"> The lexicon system presented above is fully integrated into the ELU environment; in particular, the result of analysis and the starting point for generation is the same type of feature structure as that produced by ELU grammars, and the equa- null tions within classes are of the same sort as those used elsewhere in a linguistic description, being able to exploit re-entrancy, negation, disjunction, direct manipulation of lists, etc.</Paragraph>
    <Paragraph position="1"> For the purpose of experimenting with the structure of the class hierarchy and the distribution of information within individual classes, the lexicon is held in memory, and is accessed directly by means of an exhaustive search. Once a stable description is achieved, and the coverage of the lexicon increases, a more efficient access mechanism exists, in which possible word-forms are pre-computed, and used to index into a disk file of class definitions.</Paragraph>
    <Paragraph position="2"> We have presented an implemented treatment of a framework for lexical description which is both practical from the perspective of efficiency and attractive in its reflection of the natural organization of a lexicon into nested and intersecting generalizations and exceptions. The system extends traditional unification with a multiple default inheritance mechanism, for which a declarative semantics is provided.</Paragraph>
  </Section>
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