Placing one queen at a time

Source: The Art of Prolog

Program source code: n_queens_2.pl

/*   queens(N,Queens)  :-
 
	Queens is a placement that solves the N queens problem,
 
	represented as a permutation of the list of numbers [1,2,..,N].
 
*/
 
 
 
     queens(N,Qs) :- range(1,N,Ns), queens(Ns,[],Qs).
 
 
 
     queens(UnplacedQs,SafeQs,Qs) :-
 
         select(Q,UnplacedQs,UnplacedQs1), 
 
         not attack(Q,SafeQs), 
 
         queens(UnplacedQs1,[Q|SafeQs],Qs).
 
     queens([],Qs,Qs).
 
 
 
	range(I,N,[I|Ns]) :- I < N, I1 is I+1, range(I1,N,Ns).
 
	range(N,N,[N]).
 
 
 
	select(X,[X|Xs],Xs).
 
	select(X,[Y|Ys],[Y|Zs]) :- select(X,Ys,Zs).
 
 
 
     attack(X,Xs) :- attack(X,1,Xs).
 
 
 
     attack(X,N,[Y|_]) :- X is Y+N ; X is Y-N.
 
     attack(X,N,[_|Ys]) :- N1 is N+1, attack(X,N1,Ys).
 
 
 
%	Program 14.3: Placing on
 
 queen at a time