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A Context-Free Approximation of Head-Driven Phrase Structure Grammar

Bernd Kiefer; Hans-Ulrich Krieger
In: Efficiency in Unification-Based Processing. CSLI Lecture Notes, 2001.


We present a context-free approximation of unification-based grammars, such as HPSG or PATR-II. The theoretical underpinning is established through a least fixpoint construction over a certain monotonic function. In order to reach a finite fixpoint, the concrete implementation can be parameterized in several ways, either by specifying a finite iteration depth, by using different restrictors, or by making the symbols of the CFG more complex adding annotations à la GPSG. We also present several methods that speed up the approximation process and help to limit the size of the resulting CF grammar.