nocheck(_, []).
nocheck(X/Y, [X1/Y1 | Rest]) :-
  Y =\= Y1,
  abs(Y1-Y) =\= abs(X1-X),
  nocheck(X/Y, Rest).

legal([]).
legal([X/Y | Rest]) :-
  legal(Rest),
  member(Y,[1,2,3,4,5,6,7,8]),
  nocheck(X/Y, Rest).

eightqueens(L) :-
  L = [1/_,2/_,3/_,4/_,5/_,6/_,7/_,8/_],legal(L).