See also context-free grammar, and context-sensitive grammar.

There can be situations where more than one of these is present.

For a fairly complete and quite entertaining treatment of anaphora, see Hirst, G. Anaphora in Natural Language Understanding: A Survey Springer Lecture Notes in Computer Science 119, Berlin: Springer, 1981.

In Prolog, we would write something like:

s(P1, P3, Agr) :- np(P1, P2, Agr), vp(P2, P3, Agr).

s(P1, P3, SAgr) :-
        np(P1, P2, NPAgr),
vp(P1, P2, VPAgr),
intersection(NPAgr, VPAgr, SAgr),
nonempty(SAgr).

Augmented grammar rules are also used to record sem and var features in computing logical forms, and to express the relationship between the sem and var of the left-hand side and the sem(s) and var(s) of the right-hand side. For example, for the rule vp → v (i.e. an intransitive verb), the augmented rule with sem feature could be:

vp(sem(lambda(X, ?semv(?varv, X))), var(?varv)) →

    v(subcat(none), sem(?semv), var(?varv))

where subcat none indicates that this only works with an intransitive verb.

Complex groupings of auxiliaries can occur, as in "The child may have been being taken to the movies".

Some auxiliaries (do, be, and have) can also occur as verbs in their own right.

Auxiliary verb is often abbreviated to AUX.

AUX is a lexical grammatical category.