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A metarule specifies a rule for transforming grammar rules into grammar rules.
In XTAG the grammar
rules are lexicalized trees. Hence an XTAG metarule mr
is a pair (lhs, rhs) of XTAG trees, where:
- lhs, the left-hand side of the metarule, is a pattern tree,
i.e., it is intended to present a specific pattern of tree to look for
in the trees submitted to the application of the metarule.
- When a metarule mr is applied to an input tree inp, the first
step is to verify if the input tree matches the pattern specified by
the lhs. If there is no match, the application fails.
- rhs, the right-hand side of the metarule, specifies (together
with lhs) the transformation that will be done in inp,
in case of successful matching, thus generating the output tree of
the metarule application25.1.