/* --------------------------------------------------------------
 *  "Agatha Christie" Problem  -- facts & rules but no solution
 *  Who was the killer?
 *  
 *  Alice, her husband, son, daughter, 
 *  and brother are involved in a murder. 
 *  One of the five killed one of the other
 *  four.
 *  
 *  1. A man and a woman were together in the bar
 *     at the time of the murder. 
 *  2. The victim and the killer were together 
 *     on the beach at the time of the murder. 
 *  3. One of the children was alone at the 
 *     time of the murder. 
 *  4. Alice and her husband were not together 
 *     at the time of the murder. 
 *  5. The victim's twin was innocent. 
 *  6. The killer was younger than the victim.
 *----------------------------------------------------------------------*/

/*----- Facts -----*/
person(alice).
person(husband).
person(son).
person(daughter).
person(brother).

child(son).
child(daughter).

male(husband).
male(son).
male(brother).

female(alice).
female(daughter).

twin(alice,    brother).
twin(brother,  alice).
twin(son,      daughter).
twin(daughter, son).

/*----- Rules -----*/
istwin(X) :- twin(X, _).

older(alice,   son).
older(alice,   daughter).
older(husband, son).
older(husband, daughter).

inbar(M, N) :- person(M), person(N),
               male(M),   female(N).

together(S, T) :- S=alice, T=husband.
together(S, T) :- T=alice, S=husband.

alone(P) :- person(P), child(P).