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`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.
- {Lexicalized Elementary trees
In Figure 2.4, which shows sample initial and auxiliary trees,
substitution sites are marked by a
,
and foot nodes are marked by
an .
This notation is standard and is followed in the rest of this
report.
Next: Unification-based features
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XTAG Project
http://www.cis.upenn.edu/~xtag