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<Paper uid="P06-1019">
  <Title>Sydney, July 2006. c(c)2006 Association for Computational Linguistics Partially Specified Signatures: a Vehicle for Grammar Modularity</Title>
  <Section position="5" start_page="145" end_page="145" type="relat">
    <SectionTitle>
3 Related Work
</SectionTitle>
    <Paragraph position="0"> Several authors address the issue of grammar modularization in unification formalisms. Moshier (1997) views HPSG , and in particular its signature, as a collection of constraints over maps between sets. This allows the grammar writer to specify any partial information about the signature, and provides the needed mathematical and computational capabilities to integrate the information with the rest of the signature. However, this work does not define modules or module interaction. It does not address several basic issues such as bounded completeness of the partial order and the feature introduction and upward closure conditions of the appropriateness specification. Furthermore, Moshier (1997) shows how signatures are distributed into components, but not the conditions they are required to obey in order to assure the well-definedness of the combination.</Paragraph>
    <Paragraph position="1"> Keselj (2001) presents a modular HPSG, where each module is an ordinary type signature, but each of the sets FEAT and TYPE is divided into two disjoint sets of private and public elements. In this solution, modules do not support specification of partial information; module combination is not associative; and the only channel of interaction between modules is the names of types.</Paragraph>
    <Paragraph position="2"> Kaplan et al. (2002) introduce a system designed for building a grammar by both extending and restricting another grammar. An LFG grammar is presented to the system in a priority-ordered sequence of files where the grammar can include only one definition of an item of a given type (e.g., rule) with a particular name. Items in a higher priority file override lower priority items of the same type with the same name. The override convention makes it possible to add, delete or modify rules.</Paragraph>
    <Paragraph position="3"> However, a basis grammar is needed and when modifying a rule, the entire rule has to be rewritten even if the modifications are minor. The only interaction among files in this approach is overriding of information.</Paragraph>
    <Paragraph position="4"> King et al. (2005) augment LFG with a makeshift signature to allow modular development of untyped unification grammars. In addition, they suggest that any development team should agree in advance on the feature space. This work emphasizes the observation that the modularization of the signature is the key for modular development of grammars. However, the proposed solution is ad-hoc and cannot be taken seriously as a concept of modularization. In particular, the suggestion for an agreement on the feature space undermines the essence of modular design.</Paragraph>
    <Paragraph position="5"> Several works address the problem of modularity in other, related, formalisms. Candito (1996) introduces a description language for the trees of LTAG. Combining two descriptions is done by conjunction. To constrain undesired combinations, Candito (1996) uses a finite set of names where each node of a tree description is associated with a name. The only channel of interaction between two descriptions is the names of the nodes, which can be used only to allow identification but not to prevent it. To overcome these shortcomings, Crabb'e and Duchier (2004) suggest to replace node naming by colors. Then, when unifying two trees, the colors can prevent or force the identification of nodes. Adapting this solution to type signatures would yield undesired orderdependence (see below).</Paragraph>
  </Section>
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