\begin{verbatim}

const size = 8.

col(1..size).
row(1..size).

not_in(X,Y) :-
    row(X),
    col(Y),
    not in(X,Y).

in(X,Y) :-
    row(X),
    col(Y),
    not not_in(X,Y).

has_queen(X)    :-
        row(X),
        col(Y),
        in(X,Y).

    :-  row(X),
        not has_queen(X).

    :-  row(X),
        col(Y),
        row(XX),
        not eq(X,XX),
        in(X,Y),
        in(XX,Y).

    :-  row(X),
        col(Y),
        col(YY),
        not eq(Y,YY),
        in(X,Y),
        in(X,YY).

    :-  row(X),
        col(Y),
        row(XX),
        col(YY),
        not eq(X,XX),
        not eq(Y,YY),
        in(X,Y),
        in(XX,YY),
        eq(abs(X-XX),abs(Y-YY)).

hide not_in(X,Y).
hide row(X).
hide col(Y).
hide has_queen(X).

\end{verbatim}