The phrase structure of a language L is defined in two files. The first one, called xbar.pl by convention, defines common rules. Basic rules for empty categories plus head movement. Phrase structure rules particular to language L .
Phrase structure rules for language L .
This is done before any processing occurs. Hence, care must be taken not to multiply define rules. In the following sections, we will first document the PS rules available to the user. These are used to encode families of phrase structure rules.