One major obstacle to efficient processing of large wide coverage grammars in unification-based grammatical frameworks such as HPSG is the time and space cost of the unification operation itself. In a grammar development system it is not appropriate to address this problem with techniques which involve lengthy compilation, since this slows down the edit-test-debug cycle. Nor is it possible to radically restructure the grammar. In this paper we describe novel extensions to an existing efficient unification algorithm which improve its space and time behaviour (without affecting its correctness) by substantially increasing the amount of structure sharing that takes place. We also describe a fast and automatically tunable pre-unification filter (the "quick check") which in practice detects a large proportion of unifications that if performed would fail. Finally, we present an efficient algorithm for checking for subsumption relationships between two feature structures; a special case of this gives a fast equality test. The subsumption check is used in a parser (described elsewhere in this volume) which "packs" local ambiguities to avoid performing redundant sub-computations.

The paper deals with the parsing of an HPSG grammar for German that employs the concept of word order domains and discontinuous constituents. I will propose constraints on the continuity of phrases. These constraints reduce the search space for parsing and improve the efficiency by a factor of up to 4.

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