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Grammar rules.

A grammar rule is defined by a ternary predicate, rule/3. The first argument of this predicate is a ground Prolog term indicating the rule identifier. The second argument of the rule is the mother category. Categories are non-variable Prolog terms. The third argument of the rule is a list of categories. Note that we require that the length of the list is given, and that none of the categories appearing in the list is a variable. An example of a grammar rule is provided:
...,Agr,trans,l(Arg,Sem)),np(_,Arg)]).\end{verbatim}\end{minipage}\end{displaymath} (1)

Terminal symbols cannot be introduced in rules directly, but are introduced by means of lexical entries.