% A Sample Logic Problem. [ Original logic problems, July 1990, p 5]
% Angela, David and Mae are the young stars in a talent show.
% Their ages are 5,7, and 8. One has last name Starr.
% Miss Grant is three years older than Angela.
% The child whose last name is Diamond is seven years old
%
% compile/consult this file and then run 'go.' to see the answer.
%
first(angela). first(david). first(mae).
male(david). female(angela). female(mae).
age(5). age(7). age(8).
last(diamond). last(grant). last(starr).

:-style_check(-singleton).

child(First,Last,Age):-first(First), last(Last), age(Age),
	givens(First,Last,Age),
	assertz(fact(First,Last,Age)).
% The above tries combinations of names and ages against
% the givens, and adds new data as a known fact
% when a child has two names and an age.


givens(First,Last,Age):-
	if(male(First), Last\=grant),
	if(Last=grant, female(First)),
	iff(Last=diamond,Age=7),
	iff(Last=grant,Age=8),
	iff(First=angela,Age=5),
	unknown(First, Last, Age).
% once a case has been found, assume it eliminates
% its values from other cases. 
unknown(First, Last, Age):- not( fact(First,_,_) ),
	not( fact(_,Last,_) ),
	not( fact(_,_,Age) ).

if(P,Q):- P->Q;true.
iff(P,Q):-(P,Q); not( P ), not( Q ).

:-dynamic(fact/3). % allow new facts to be added.

go:-child(First, Last, Age), fail; listing(fact).
% The 'fail' above forces Prolog to try and find another child.
% When all the children are found prolog takes the next branch after
%	the semicolon and lists the facts it has filed.

:- write('Puzzle loaded'), nl.
:- write('Input go. to start solving it...'), nl.