Separating Surface Order and Syntactic Relations in a Dependency Grammar

Separating Surface Order and Syntactic Relations in a Dependency Grammar Norbert BrSker Universitiit Stuttgart Azenbergstr. 12 D-70174 Stuttgart NOBI~IMS. UNI-STUTTGART. DE Abstract This paper proposes decoupling the dependency tree from word order, such that surface ordering is not determined by traversing the dependency tree. We develop the notion of a word order do- main structure, which is linked but structurally dissimilar to the syntactic dependency tree. The proposal results in a lexicalized, declarative, and formally precise description of word order; fea- tures which lack previous proposals for depen- dency grammars. Contrary to other lexicalized approaches to word order, our proposal does not require lexical ambiguities for ordering alterna- tives. 1 Introduction Recently, the concept of valency has gained con- siderable attention. Not only do all linguis- tic theories refer to some reformulation of the traditional notion of valency (in the form of 0- grid, subcategorization list, argument list, or ex- tended domain of locality); there is a growing number of parsers based on binary relations be- tween words (Eisner, 1997; Maruyama, 1990). Given this interest in the valency concept, and the fact that word order is one of the main difference between phrase-structure based approaches (henceforth PSG) and dependency grammar (DG), it is valid to ask whether DG can capture word order phenomena without re- course to phrasal nodes, traces, slashed cate- gories, etc. A very early result on the weak generative equivalence of context-free grammars and DGs suggested that DGs are incapable of describing surface word order (Gaifman, 1965). This result has recently been critizised to apply only to impoverished DGs which do not properly represent formally the expressivity of contempo- rary DG variants (Neuhaus & Br6ker, 1997). Our position will be that dependency re- lations are motivated semantically (Tesni~re, 1959), and need not be projective (i.e., may cross if projected onto the surface ordering). We argue for so-called word order domains, consist- ing of partially ordered sets of words and associ- ated with nodes in the dependency tree. These order domains constitute a tree defined by set in- clusion, and surface word order is determined by traversing this tree. A syntactic analysis there- for consists of two linked, but dissimilar trees. Sec. 2 will briefly review approaches to word order in DG. In Sec. 3, word order domains will be defined, and Sec. 4 introduces a modal logic to describe dependency structures. Sec. 5 ap- plies our approach to the German clause and Sec. 6 relates it to some PSG approaches. 2 Word Order in DG A very brief characterization of DG is that it recognizes only lexical, not phrasal nodes, which are linked by directed, typed, binary rela- tions to form a dependency tree (Tesni~re, 1959; Hudson, 1993). The following overview of DG flavors shows that various mechanisms (global rules, general graphs, procedural means) are generally employed to lift the limitation of pro- jectivity and discusses some shortcomings of these proposals. Functional Generative Description (Sgall et al., 1986) assumes a language-independent underlying order, which is represented as …

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