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Feature Matching

In the previous subsections we have considered only the aspects of a metarule involving the structural part of the XTAG trees. In a feature based grammar as XTAG is, accounting for features is essential. A metarule is not really worth if it doesn't account for the proper change of feature equations25.9 from the input to the output tree. The aspects that have to be considered here are: Based on the possible combinations of these requirements we partition the feature equations into the following five classes25.10: Typed variables can be used in feature equations in both lhs and rhs. They are intended to represent the nodes of the input tree to which they have been instantiated. For each resulting match from the structural matching process the following is done: Finally, a new kind of metavariable, which is not used at the nodes, can be introduced in the feature equations part. They have the same form of the non-typed variables, i.e. quotation mark, followed by a number, and are used in the place of feature values and feature names. Hence, if the equation ?NP?.b:<?2> = ?3 appears in lhs, this means, that all feature equations of inp that match a bottom attribute of some NP to any feature value (but not to a feature path) will not be copied to the output.
next up previous contents
Next: Examples Up: The definition of a Previous: Output Generation
XTAG Project
1998-09-14