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 equations^25.9 from the input to the output tree. The aspects that have to be considered here are:

• Which feature equations should be required to be present in inp in order for the match to succeed.
• Which feature equations should be generated in the output tree as a function of the feature equations in the input tree.

Based on the possible combinations of these requirements we partition the feature equations into the following five classes^25.10:

• Require & Retain: Feature equations in this class are required to be in inp in order for matching to succeed. Upon matching, these equations will be copied to the output tree. To achieve this behaviour, the equation must be placed in the lhs tree of the metarule preceded by a plus character (e.g. +V.t:<trans>=+)^25.11
• Require & Don't Copy: The equation is required to be in inp for matching, but should not be copied to the output tree. Those equations must be in lhs preceded by minus character (e.g. -NP₁:<case>=acc).
• Optional & Don't Copy: The equation is not required for matching, but we have to make sure not to copy it to the output tree set of equations, regardless of it being present or not in inp. Those equations must be in lhs in raw form, i.e. neither preceded by a plus nor minus character (e.g. S_r.b:<perfect>=VP.t:<perfect>).
• Optional & Retain: The equation is not required for matching but, in case it is found in inp it must be copied to the output tree. This is the default case, and hence these equations should not be present in the metarule specification.
• Add: The equation is not required for matching but we want it to be put in the output tree anyway. These equations are placed in raw form in the rhs (notice in this case it is the right hand side).

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:

• The (typed) variables in the equations at lhs and rhs are substituted by the names of the nodes they have been instantiated to.
• The requirements concerning feature equations are checked, according to the above rules.
• If the match survives feature equation checking, the proper output tree is generated, according to Section B.2.3 and to the rules described above for the feature equations.

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.