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As modifiers, adjectives anchor the tree shown in Figure 20.1. The features of the N node onto which the An tree adjoins are passed through to the top node of the resulting N. The null adjunction marker (NA) on the N foot node imposes right binary branching such that each subsequent adjective must adjoin on top of the leftmost adjective that has already adjoined. Due to the NA constraint, a sequence of adjectives will have only one derivation in the XTAG grammar. The adjective's morphological features such as superlative or comparative are instantiated by the morphological analyzer. See Chapter 23 for a description of how we handle comparatives. At this point, the treatment of adjectives in the XTAG English grammar does not include selectional or ordering restrictions. Consequently, any adjective can adjoin onto any noun and on top of any other adjective already modifying a noun. All of the modified noun phrases shown in ((339))-((342)) currently parse with the same structure shown for colorless green ideas in Figure 20.2.
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While ((340))-((342)) are all semantically anomalous, ((342)) also suffers from an ordering problem that makes it seem ungrammatical in the absence of any licensing context. One could argue that the grammar should accept ((339))-((341)) but not ((342)). One of the future goals for the grammar is to develop a treatment of adjective ordering similar to that developed by [#!ircs:det98!#] for determiners20.2. An adequate implementation of ordering restrictions for adjectives would rule out ((342)).