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<Paper uid="W98-0507">
  <Title>I ! I I</Title>
  <Section position="3" start_page="0" end_page="58" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> Much recent work in linguistics and computational linguistics emphasizes the role of lexical information in syntactic representation and processing.</Paragraph>
    <Paragraph position="1"> This emphasis given to the lexicon is the result of a gradual process. The original trend in linguistics has been to individuate categories of words having related characteristics - the traditional syntactic categories like verb, noun, adjective, etc. - and to express the structure of a sentence in terms of constituents, or phrases, built around these categories.</Paragraph>
    <Paragraph position="2"> Subsequent considerations lead to a lexicalization of grammar. Linguistically, the constraints expressed on syntactic categories are too general to explain facts about words - e.g. the relation between a verb and its nominalization, &amp;quot;destroy the city&amp;quot; and &amp;quot;destruction of the city&amp;quot; - or to account uniformly for a number of phenomena across languages - e.g. passivization. In parsing, the use of individual item information reduces the search space of the possible structures of a sentence. From a mathematical point of view, lexicalized grammars exhibit properties - like finite ambiguity (Schabes, 1990) - that are of a practical interest (especially in writing realistic grammars). Dependency grammar is naturally suitable for a lexicalization, as the binary relations representing the structure of a sentence are defined with respect to the head (that is a word).</Paragraph>
    <Paragraph position="3"> Pure lexicalized formalisms, however, have also several disadvantages. Linguistically, the abstract level provided by syntactic rules is necessary to avoid the loss of generalization which would arise if class-level information were repeated in all lexical items. In parsing, a predictive component is required to guarantee the valid prefiz property, namely the capabifity of detecting as soon as possible whether a substring is a valid prefix for the language defined by the grammar. Knowledge of syntactic categories, which does not depend on the input, is needed for a parser to be predictive.</Paragraph>
    <Paragraph position="4"> In this paper we address the problem of the interaction between syntactic and lexical information in dependency grammar. We introduce many intermediate levels between lexical items and syntactic categories, by organizing the grammar around the notion of subcategorizetion. Intuitively, a subcategorization frame for a lexical item L is a specification of the number and type of elements that L requires in order, for ml utterance that contains L, to be well-formed. For example, within the syntactic category VERB, different verbs require different numbers of nominal dependents for a well-formed sentence. In Italian (our case study), an intransitive verb such as dormirv, &amp;quot;sleep&amp;quot;, subcategorizes for only one nominal element (the subject), while a transitive verb such as baciare, &amp;quot;kiss&amp;quot;, subcategorizes for two nominal elements (the subject and the object)  1. Grammatical relations such as subject and object  are primitive concepts in a dependency paradigm, i.e. they directly define the structure of the sentence. Consequently, the dependency paradigm is particularly suitable to define the grammar in terms of constraints on subcategorization frames.</Paragraph>
    <Paragraph position="5"> Our proposal is to use subcategories organized in a hierarchy: the upper level of the hierarchy corresponds to the syntactic categories, the other levels correspond to subcategories that are more and more 1We include the subject relation in the subcategorization, or valency, of a verb - cf. (Hudson, 1990) (Mel'cuk, 1988). In most constituency theories, on the contrary, the subject is not part of the valency of a verb.</Paragraph>
    <Paragraph position="6">  specific as one descends the hierarchy. This representation is advantageous because of its compactness, and bemuse the hierarchical mixed-grained organization of the information is useful in processing. In fact, using the general knowledge at the upper level of the hierarchy, we can make predictions on the structure of the sentence before encountering the lexical head.</Paragraph>
    <Paragraph position="7"> Hierarchical formalisms have been proposed in some theories. Pollard and Sag (1987) suggested a hierarchical organization of lexical information: as far as subcategorization is concerned, they introduced a &amp;quot;hierarchy of lexical types&amp;quot;. A specific formalisation of this hierarchy has never reached a wide consensus in the HPSG community, but several proposals have been developed - see for example (Meurers, 1997), that uses head subtypes and lexical principles to express generalizations on the valency properties of words.</Paragraph>
    <Paragraph position="8"> Hudson (1990) adopts a dependency approach and uses hierarchies to organize different kinds of linguistic information, for instance a hierarchy including word classes and lexical items. The subcategorization constraints, however, are specified for each lexical item (for instance STAND -4 STAND-intrans, STAND-trans): this is highly redundant and misses important generalizations.</Paragraph>
    <Paragraph position="9"> In LTAG (Joshi and Schabes, 1996), pure syntactic information is grouped around shared subcategorization constraints (tree families). Hierarchical representations of LTAG have been proposed: (Vijay-Shanker and Schabes, 1992), (Becker, 1993), (Evans et al., 1995), (Candito, 1996), (Doran et al., 1997).</Paragraph>
    <Paragraph position="10"> However, none of these works proposes to use the hierarchical representation in processing - just Vijay-Shanker and Schabes (1992) mention, as a possible future investigation, the definition of parsing strategies that take advantage of the hierarchical representation. null The goal of our hierarchical formalism is twofold.</Paragraph>
    <Paragraph position="11"> On one side, we want to provide a hierarchical organization to a lexicalized dependency formalism: similarly to the hierarchical representations of LTAG, the aim is to solve the problems of redundancy and lexicon maintenance of pure lexicalized approaches.</Paragraph>
    <Paragraph position="12"> On the other side, we want to explore how a hierarchical formAllgm can be used in processing in order to get the maximum benefit from it.</Paragraph>
    <Paragraph position="13"> The paper is organized as follows: in section 2 we describe a lexiealized dependency formalism that is a simplified version of (Lombardo and Lesmo, 1998).</Paragraph>
    <Paragraph position="14"> Starting from this formalism, we define in section 3 the hierarchy of subcategories. In section 4, we sketch a parsing model that uses the hierarchical grammar. In section 5, we describe an application of the formalism to the classification of 101 Italian verbs. Section 6 concludes the paper.</Paragraph>
  </Section>
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