In developing the OVIS grammar we have tried to combine the short-term goal of developing a grammar which meets the requirements imposed by the application (i.e. robust processing of the output of the speech recognizer, extensive coverage of locative phrases and temporal expressions, and the construction of fine-grained semantic representations) with the long-term goal of developing a general, computational, grammar which covers all the major constructions of Dutch.
The grammar currently covers the majority of verbal subcategorization types (intransitives, transitives, verbs selecting a PP, and modal and auxiliary verbs), NP-syntax (including pre- and postnominal modification, with the exception of relative clauses), PP-syntax, the distribution of VP-modifiers, various clausal types (declaratives, yes/no and WH-questions, and subordinate clauses), all temporal expressions and locative phrases relevant to the domain, and various typical spoken-language constructs. Due to restrictions imposed by the speech recognizer, the lexicon is relatively small (2000 word forms, most of which are names of stations and cities).
From a linguistic perspective, the OVIS-grammar can be characterized as a constraint-based grammar, which makes heavy use of (multiple) inheritance. As the grammar assumes quite complex lexical signs, inheritance is absolutely essential for organizing the lexicon succinctly. However, we not only use inheritance at the level of the lexicon (which is a well-known approach to computational lexica), but have also structured the rule-component using inheritance.
An important restriction imposed by the grammar-parser interface is that rules must specify the category of their mothers and daughters. That is, each rule must specify the type of sign of its mother and daughters. A consequence of this requirement is that general rule-schemata, as used in Categorial Grammar and HPSG cannot be used directly in the OVIS grammar. A rule which specifies that a head daughter may combine with a complement daughter, if this complement unifies with the first element on SUBCAT of the head (i.e. a version of the categorial rule for functor-argument application) cannot be implemented directly, as it leaves the categories of the daughters and mother unspecified. Nevertheless, capturing generalizations of this type does seem desirable.
We have therefore adopted an architecture for grammar rules similar to that of HPSG [8], in which individual rules are classified in various structures, which are in turn defined in terms of general principles. For instance, the grammar currently contains several head-complement rules (which allow a verb, preposition, or determiner to combine with one or more complements). These rules need only specify category-information and the relative order of head and complement(s). All other information associated with the rule (concerning the matching of head-features, the instantiation of features used to code long-distance dependencies, and the semantic effect of the rule) follows from the fact that the rules are instances of the class head-complement structure. This class itself is defined in terms of general principles, such as the head-feature, valence, filler and semantics principle. Other rules are defined in terms of the classes head-adjunct and head-filler structure, which in turn inherit from (a subset of) the general principles mentioned above. Thus, even though the grammar contains a relatively large number of rules (compared to lexicalist frameworks such as HPSG and CG), the redundancy in these rules is minimal.
The resulting grammar has the interesting property that it combines the strong tendency towards lexicalism and positing general combinatoric rule schemata present in frameworks such as HPSG with relatively specific grammar rules to facilitate efficient processing.