% The Famous Barber's paradox
% In a certain village, there is a barber who shaves
% those residents who do not shave themselves,
% and all residents who don't shave themselves
% are shaved by the barber.

:-op(600, xfx, shaves).
resident(X):-(X=jim ;  X=john ;  X=barber).
shaving(X):-
	barber shaves X ;  X shaves X.
barber shaves X:-!,resident(X), X\=barber, print(X), write(' can be shaved by the barber'), nl.
X shaves X:-!, resident(X), X\=barber, print(X), write(' can shave himself'), nl.
X shaves Y:-print(X), write(' shaves '), print(Y),  write('............NOT!'), nl, fail.

go:-resident(X),shaving(X),fail.
paradox:-trace, resident(Who), Who shaves barber.