nocheck(_, []).
nocheck(X/Y, [X1/Y1 | Rest]) :-
  % X =\= X1, assume the Xs are distinct
  Y =\= Y1,
  abs(Y1-Y) =\= abs(X1-X),
  nocheck(X/Y, Rest).

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

/*
  eightqueens(X) succeeds if X is a legal
  placement of eight queens, listed in order
  of their X coordinates.
*/
eightqueens(X) :-
  X = [1/_,2/_,3/_,4/_,5/_,6/_,7/_,8/_],
  legal(X).