% grammar rules:

s(P1,P3,s(NP,VP)) :- np(P1,P2,NP), vp(P2,P3,VP).
vp(P1,P3,vp(v(Verb),NP)) :- v(P1,P2,Verb), np(P2,P3,NP).
np(P1,P2,np(name(Name))) :- proper(P1,P2,Name).
np(P1,P3,np(det(Det),noun(Noun))) :- det(P1,P2,Det), noun(P2,P3,Noun).

% lexicon entries:

isname(john).
isverb(fed).
isdet(the).
isnoun(numbat).

% rules to do lexical analysis - that is to classify words according
% to their part of speech:

det(From, To, Word) :- word(Word, From, To), isdet(Word).
noun(From, To, Word) :- word(Word, From, To), isnoun(Word).
v(From, To, Word) :- word(Word, From, To), isverb(Word).
proper(From, To, Word) :- word(Word, From, To), isname(Word).

% An example sentence to parse:

word(john, 1, 2).
word(fed, 2, 3).
word(the, 3, 4).
word(numbat, 4, 5).

% to use this program, save it in your own directory, say under
% the name dcg2.pl, then do:
%
% % prolog -s dcg2.pl
% ?- s(Start, Finish, Parse).
%
% The result should tell you Start = 1, Finish = 5, and Parse = a parse of the
% sentence. See if you can extend the lexicon and maybe the grammar
% so that you can parse some other sentences.