#const n = 20.
#const k = 5.

% n-Queens encoding

{ q(I,1..n) } == 1 :- I = 1..n.
{ q(1..n,J) } == 1 :- J = 1..n.
:- { q(D-J,J) } >= 2, D =   2..2*n.
:- { q(D+J,J) } >= 2, D = 1-n..n-1.
:- q(X1, Y1), q(X2, Y2), X1 != X2, |(X1 - X2)| + |(Y1 - Y2)| <= k.