s(P1,P3) :- np(P1,P2), vp(P2,P3).
np(P1,P2) :- propernoun(P1,P2).
vp(P1,P2) :- verb(P1,P2).
vp(P1,P3) :- verb(P1,P2), adverb(P2,P3).

% Terminal symbols

propernoun(I,O) :- word(W,I,O), ispropernoun(W).
verb(I,O) :- word(W,I,O), isverb(W).
adverb(I,O) :- word(W,I,O), isadverb(W).
adjective(I,O) :- word(W,I,O), isadverb(W).

ispropernoun(bill).   ispropernoun(steve).
isverb(cries).        isverb(smiles).
isadverb(often).      isadverb(slightly).
isadjective(red).     isadjective(sad).

% Encoding of the sentence "Bill cries often"

word(bill,1,2).
word(cries,2,3).
word(often,3,4).