Crossed Serial Dependencies: A low-power parseable extension to GPSG

Crossed Serial Dependencies: i low-power parseable extension to GPSG Henry Thompson Department of Artificial Intelligence and Program in Cognitive Science University of Edinburgh Hope Park Square, Meadow Lane Edinburgh EH8 9NW SCOTLAND ABSTRACT An extension to the GPSG grammatical formalism is proposed, allowing non-terminals to consist of finite sequences of category labels, and allowing schematic variables to range over such sequences. The extension is shown to be sufficient to provide a strongly adequate grammar for crossed serial dependencies, as found in e.g. Dutch subordinate clauses. The structures induced for such constructions are argued to be more appropriate to data involving conjunction than some previous proposals have been. The extension is shown to be parseable by a simple extension to an existing parsing method for GPSG. I. INTRODUCTION There has been considerable interest in the community lately with the implications of crossed serial dependencies in e.g. Dutch subordinate clauses for non-transformational theories of grammar. Although context-free phrase structure grammars under the standard interpretations are weakly adequate to generate such languages as anb n, they are not capable of assigning the correct dependencies - that is, they are notstrongly adequate. In a recent paper (Bresnan Kaplsn Peters end Zaenen 1982) (hereafter BKPZ), a solution to the Dutch problem was presented in terms of LFG (Kaplan and Bresnan 1982), which is known to have considerably more than context-free power. (Steedman 1983) and (Joshi 1983) have also made proposals for solutions in terms of Steedman/Ades grammars and tree adjunction grammars (Ades and Steedman 1982; Joshi Levy and Yueh 1975). In this paper I present a minimal extension to the GPSC formalism (Gazdar 1981c) which also provides a solution. It induces structures for the relevant sentences which are non-trivially distinct from those in BKPZ, and which I argue are more appropriate. It appears, when suitably constrained, to be similar to Joshi's proposal in making only a small increment in power, being incapable, for instance, of analysing anbnc n with crossed dependencies. And it can easily be parsed by a small modification to the parsing mechanisms I have already developed for GPSG. II. AN EXTENSION TO GPSG II.I Extendin G the s~ntax GPSG includes the idea of compound non-terminals, composed of pairs of standard category labels. We can extend this trivially to finite sequences of category labels. This in itself does not change the weak generative capacity of the grammar, as the set of non-terminals remains finite. CPSG also includes the idea of rule schemata - rules with variables over categories. If we further allow variables over sequences, then we get a real change. At this point I must introduce some notation. I will write [a,b ,c] for a non-terminal label composed of the categories a, b, and c. I will write Za b* to indicate that the schematic variable Z ranges over sequences of the category b. We can then give the following grammar for anb n with crossed 16 dependencies: S -> e S:Z -> a SIZ:b .(I) s:z -> a s z:b (2) …

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