Introduction

XTAG has now a collection of functions accessible from the user interface that helps the user in the construction and maintenance of a tag tree-grammar. This subsystem is based on the idea of metarules ([#!becker93!#]). Here our primary purpose is to describe the facilities implemented under this metarule-based subsystem. For a discussion of the metarules as a method for compact representation of the Lexicon see [#!becker93!#] and [#!srini94!#]. The basic idea of using metarules is to take profit of the similarities of the relations involving related pairs of XTAG elementary trees. For example, in the English grammar described in this technical report, comparing the XTAG trees for the basic form and the wh-subject moved form, the relation between this two trees for transitive verbs ( , ) is similar to the relation for the intransitive verbs ( , ) and also to the relation for the ditransitives ( , ). Hence, instead of generating by hand the six trees mentioned above, a more natural and robust way would be generating by hand only the basic trees for the intransitive, transitive and ditransitive cases, and letting the wh-subject moved trees to be automatically generated by the application of a unique transformation rule that would account exactly for the identical relation involved in each of the three pairs above. Notice that the degree of generalization can be much higher than it might be thought in principle from the above paragraph. For example, once a rule for passivization is applied to the tree different basic trees above, the wh-subject moved rule could be again applied to generate the wh-moved subject versions for the passive form. Depending on the degree of regularity that one can find in the grammar being built, the reduction in the number of original trees can be exponential. We still make here a point that the reduction of effort in grammar construction is not the only advantage of the approach. Robustness, reliability and maintainability of the grammar achieved by the use of metarules are equally or even more important. In the next section we define a metarule in XTAG. Section 3 gives some linguistically motivated examples of metarule for the English grammar described in this technical report and their application. Section 4 describes the access through the user interface.