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Elementary trees in TAGs capture several notions of locality, with the most
primary of these being locality of -role assignment. Each elementary
tree has associated with it the -roles assigned by the anchor of that
elementary tree. In the current XTAG system, while the notion of locality of
-role assignment within an elementary tree has been implicit, the
-roles assigned by a head have not been explicitly represented in the
elementary tree. Incorporating -role information will make the
elementary trees more informative and will enable efficient pruning of spurious
derivations when embedded into a specific context. In the case of a
Synchronous TAG, -roles can also be used to automatically establish
links between two elementary trees, one in the object language and one in the
target language.
XTAG Project
http://www.cis.upenn.edu/~xtag