% From the book
% PROLOG PROGRAMMING IN DEPTH
% by Michael A. Covington, Donald Nute, and Andre Vellino
% (Prentice Hall, 1997).
% Copyright 1997 Prentice-Hall, Inc.
% For educational use only

% File AGREEMNT.PL
% Illustration of grammatical agreement features

% The argument N is the number of
% the subject and main verb.
% It is instantiated to 'singular'
% or 'plural' as the parse progresses.

sentence --> noun_phrase(N), verb_phrase(N).

noun_phrase(N) --> determiner(N), noun(N).

verb_phrase(N) --> verb(N), noun_phrase(_).
verb_phrase(N) --> verb(N), sentence.

determiner(singular) --> [a].
determiner(_)        --> [the].
determiner(plural)   --> [].

noun(singular) --> [dog];[cat];[boy];[girl].
noun(plural)   --> [dogs];[cats];[boys];[girls].

verb(singular) --> [chases];[sees];[says];[believes].
verb(plural)   --> [chase];[see];[say];[believe].