The application of two-level morphology to non-concatenative German morphology

In this paper we describe a hybrid system for morphological analysis and synthesis. We call it hybrid because it consists of two separate parts interacting with each other in a well-defined way. The treatment of morphonology and non-concatenative morphology is based on the two-level approach originally proposed by Koskenniemi (1983). For the concatenative part of morphosyntax (i.e. affixation) we make use of a grammar based on feature-unification. Both parts rely on the same morph lexicon.Combinations of two-level morphology with feature-based morphosyntactic grammars have already been proposed by several authors (c.f. Bear 1988a, Carson 1988, Gorz & Paulus 1988, Schiller & Steffens 1990) to overcome the shortcomings of the continuation-classes originally proposed by Koskenniemi (1983) and Karttunen (1983) for the description of morphosyntax. But up to now no linguistically satisfying solution has been proposed for the treatment of non-concatenative morphology in such a framework. In this paper we describe an extension to the model which will allow for the description of such phenomena. Namely we propose to restrict the applicability of two-level rules by providing them with filters in the form of feature structures. We demonstrate how a well-known problem of German morphology, so-called "Umlautung", can be described in our approach in a linguistically motivated and efficient way.