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