Dealing with optional complements in HPSG-based grammar implementations

Abstract

This paper discusses how the English Resource Grammar (ERG) captures the optionality of certain complements of verbs based on a single lexical entry coupled with an ontology of markings distinguishing optional from obligatory as well as unrealized from realized elements. Subject-head and head-complement structures are modified accordingly, but due to the lack of a possibility to express and use relational goals in grammars implemented in the LKB system, the ERG encoding falls short of the goal of treating optional complements in a general way. Instead, it requires two new types of ˋauxiliary' phrases which are otherwise unmotivated. We show that the problem can be overcome by using a recursive relation selecting a member from a list. The use of a lean implementation platform not supporting such relational goals, such as the LKB, thus results in a loss of generality of the grammars that can be expressed, which undermines the closeness of the implemented grammar to current linguistic analyses as one of the hallmarks of HPSG-based grammar implementation. The case study presented in this paper thus supports the position argued in Götz and Meurers (1997) that a system for the implementation of HPSG-based grammars should include both universal implicational principles as well as definite clauses over feature terms.