PRO in a unification based framework

Tensed forms of a verb assign nominative case, and untensed forms assign case none, as the progressive form of the verb sing does in Figure 4.6(b). This is different than assigning no case at all, as one form of the infinitive marker to does. See Section 8.5 for more discussion of this special case.) The distinction of a case none from no case is indicative of a divergence from the standard GB theory. In GB theory, the absence of case on an NP means that only PRO can fill that NP. With feature unification as is used in the FB-LTAG grammar, the absence of case on an NP means that any NP can fill it, regardless of its case. This is due to the mechanism of unification, in which if something is unspecified, it can unify with anything. Thus we have a specific case none to handle verb forms that in GB theory do not assign case. PRO is the only NP with case none. Note that although we are drawn to this treatment by our use of unification for feature manipulation, our treatment is very similar to the assignment of null case to PRO in [#!ChomskyLasnik93!#]. [#!watanabe93!#] also proposes a very similar approach within Chomsky's Minimalist framework.^4.10