Lexicalization and Generative Power in CCG

Abstract

The weak equivalence of Combinatory Categorial Grammar (CCG) and Tree-Adjoining Grammar (TAG) is a central result of the literature on mildly context-sensitive grammar formalisms. However, the version of CCG for which this equivalence has been established differs significantly from the version that is in use today. In particular, it allows to restrict combinatory rules on a per grammar basis, while modern CCG assumes a universal set of rules, isolating all cross-linguistic variation in the lexicon. In this article we investigate the formal significance of this difference. Our main result is that current lexicalized forms of CCG are strictly less powerful than TAG.