% Eight queens problem, with tester pushed completely inside the
% generator permu.

queens(Qs) :- permu_test([1,2,3,4,5,6,7,8],Qs,1,[],[]).

permu_test([],[],_,_,_).
permu_test(Qs,[Y|Ys],X,Cs,Ds) :- 
	del(Y,Qs,Rs), 
	C is X-Y, \+ memberchk(C,Cs),
	D is X+Y, \+ memberchk(D,Ds),
	X1 is X+1,
	permu_test(Rs,Ys,X1,[C|Cs],[D|Ds]).

del(X,[X|Xs],Xs).
del(X,[Y|Ys],[Y|Zs]) :- del(X,Ys,Zs).