Eight Queens solver

%---
% Revision 3 of the queens solver.
%---

% A solved board is a board that has a piece in every row, and no piece
% intersects any other (no rows are shared and no diagonals are shared).
solvedBoardPerm(X) :- boardPerm(X), noIntersect(X).
boardPerm([A,B,C,D,E,F,G,H]) :- row(A),row(B),row(C),row(D),row(E),row(F),row(G),row(H).
row(A) :- between(0,7,A).

% The head doesn't intersect anything in the tail, and there are no
% intersections in the tail.
noIntersect([]).
noIntersect([Head | Tail]) :- noIntersect(Head, Tail, 1), noIntersect(Tail).

noIntersect(_, [], _).
noIntersect(Ele, [Head | Tail], DiagOffset) :-
  Ele \= Head, % The row is not shared.
  PosDiag is Head - DiagOffset,  Ele \= PosDiag, % The positive diagonal is not shared.
  NegDiag is Head + DiagOffset,  Ele \= NegDiag, % The negative diagonal is not shared.
  % The element doesn't intersect anything in the tail.
  NextDiagOffset is DiagOffset + 1, noIntersect(Ele, Tail, NextDiagOffset).