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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
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