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code/asp/pqueens.lp 575 Bytes
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808facfe   Francisco Coelho   Main text adapted... 
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% Column and row domains.
col(1 .. n).
row(1 .. n).
% One queen on some row per colum.
1 { queen(I, J) : row(J) } 1 :- col(I).
% One queen on some columns per row.
1 { queen(I, J) : col(I) } 1 :- row(J).
% Number descending diagonals.
diag1(I, J, I - J + n) :- col(I), row(J).
% Number ascending diagonals.
diag2(I, J, I + J - 1) :- col(I), row(J).
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%
% Negative Restrictions
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:- D = 1 .. 2 * n - 1, not { queen(I, J) : diag1(I, J, D) } 1.
:- D = 1 .. 2 * n - 1, not { queen(I, J) : diag2(I, J, D) } 1.
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%
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% Output this predicate.
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#show queen/2.
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%#show diag1/3.
%#show diag2/3.