solution( Queens ) :- permutation( [1,2,3,4,5,6,7,8], Queens),
                      safe( Queens ).

safe( [] ).
safe( [ Queen | Others ] ) :- safe( Others ),
                              noattack( Queen, Others, 1 ).

noattack( _, [], _ ).
noattack( Y, [ Y1 | Yrest ], Xdist ) :-
                Y1-Y =\= Xdist,
                Y-Y1 =\= Xdist,
                Dist1 is Xdist + 1,
                noattack( Y, Yrest, Dist1 ).