:- use_module(library(clpfd)).

queens([E1,E2,E3,E4,E5,E6,E7,E8]) :-
	E1 in 1..8,
	E2 in 1..8,
	E3 in 1..8,
	E4 in 1..8,
	E5 in 1..8,
	E6 in 1..8,
	E7 in 1..8,
	E8 in 1..8,
	safe([E1,E2,E3,E4,E5,E6,E7,E8]),
	labeling([ffc],[E1,E2,E3,E4,E5,E6,E7,E8]).

notattack(X,Xs) :- notattack(X,Xs,1).
notattack(X,[],N).
notattack(X,[Y|Ys],N) :- X #\= Y, 
			 X #\= Y - N, 
			 X #\= Y + N,
			 N1 is N + 1,
			 notattack(X,Ys,N1).

safe([]).
safe([F|T]) :- notattack(F,T), safe(T).