s(P0, P) :- 
      np(P0, P1), 
      vp(P1, P).

np(P0, P) :-
      pn(P0, P).

np(P0, P) :-
      det(P0, P1),
      n(P1, P2),
      optrel(P2, P).

vp(P0, P) :-
      tv(P0, P1),
      np(P1, P).

vp(P0, P) :-
      iv(P0, P).

optrel(P,P).

optrel(P0,P) :-
      connects(that, P0, P1), vp(P1,P).

pn(P0, P) :- connects(harry, P0, P).
pn(P0, P) :- connects(barry, P0, P).

iv(P0, P) :- connects(walks, P0, P).

tv(P0, P) :- connects(sees, P0, P).

det(P0, P) :- connects(the, P0, P).

n(P0, P) :- connects(frog, P0, P).

/* tedious statement of sentence */

connects(harry, 0, 1).
connects(sees, 1, 2).
connects(the, 2, 3).
connects(frog, 3, 4).
connects(that, 4, 5).
connects(walks, 5, 6).

%| ?- [dcg0].
%% consulting /amd/nfs/pegasus/disk/ptn053/steedman/lectures/tl/dcg0...
%% consulted /amd/nfs/pegasus/disk/ptn053/steedman/lectures/tl/dcg0 in module user, 0 msec -88 bytes
%yes
%| ?- s(0,6).
%yes
%| ?- s(0,5).
%no
%| ?- s(0,4).
%yes
%| ?-