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.

Deutsches Forschungszentrum für Künstliche Intelligenz
German Research Center for Artificial Intelligence