The definition of a metarule in XTAG

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 application^25.1.