Lexicalization

`Lexicalized' grammars systematically associate each elementary structure with a lexical anchor. This means that in each structure there is a lexical item that is realized. It does not mean simply adding feature structures (such as head) and unification equations to the rules of the formalism. These resultant elementary structures specify extended domains of locality (as compared to CFGs) over which constraints can be stated. Following [#!schabes88!#] we say that a grammar is LEXICALIZED if it consists of 1) a finite set of structures each associated with a lexical item, and 2) an operation or operations for composing the structures. Each lexical item will be called the ANCHOR of the corresponding structure, which defines the domain of locality over which constraints are specified. Note then, that constraints are local with respect to their anchor. Not every grammar is in a lexicalized form.^2.3 In the process of lexicalizing a grammar, the lexicalized grammar is required to be strongly equivalent to the original grammar, i.e. it must produce not only the same language, but the same structures or tree set as well.

(a)		(b)		(c)		(d)

{Lexicalized Elementary trees

 

In Figure 2.4, which shows sample initial and auxiliary trees, substitution sites are marked by a $\downarrow$, and foot nodes are marked by an $\ast$. This notation is standard and is followed in the rest of this report.