/*
  nocheck(X/Y,L) takes a queen X/Y and a list
  of queens.  We succeed if and only if the X/Y
  queen holds none of the others in check.
*/
nocheck(_, []).
nocheck(X/Y, [X1/Y1 | Rest]) :-
  X =\= X1,
  Y =\= Y1,
  abs(Y1-Y) =\= abs(X1-X),
  nocheck(X/Y, Rest).

/*
  legal(L) succeeds if L is a legal placement of
  queens: all coordinates in range and no queen
  in check.
*/
legal([]).
legal([X/Y | Rest]) :-
  legal(Rest),
  member(X,[1,2,3,4,5,6,7,8]),
  member(Y,[1,2,3,4,5,6,7,8]),
  nocheck(X/Y, Rest).