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  1.  
  2.  
  3. queens(N, Qs) :- length(Qs, N), Qs ins 1..N, safeColumn(Qs), labeling([ffc], Qs).
  4.  
  5. safeColumn([]).
  6. safeColumn([Q|Qs]) :- safeColumn(Qs, Q, 1), safeColumn(Qs).
  7.  
  8. safeColumn([],_,_).
  9. safeColumn([Q|Qs], SelQ, Diagoff) :- SelQ #\= Q, abs(SelQ - Q) #\= Diagoff, Diagoff1 #= Diagoff + 1, safeColumn(Qs, SelQ, Diagoff1).
Success #stdin #stdout #stderr 0.03s 6904KB
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stdin
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queens(6, L).
stdout
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Standard output is empty
stderr
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ERROR: /home/zZWsku/prog:3:35: Syntax error: Operator expected
ERROR: /home/zZWsku/prog:9:42: Syntax error: Operator expected
ERROR: '$runtoplevel'/0: Undefined procedure: program/0
   Exception: (3) program ? EOF: exit
https://ideone.com/3gM0dZ
language:
Prolog (swi 7.6.4)
created:
185 days ago
visibility:
public

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