common lisp - How do you print only the first solution of backtracking? -

i working on n-queens problem using backtracking in lisp. far, code prints possible solutions n>=4. however, wish print first solution value of n.

(defun backtracksearch (row n) // (if () *i need line here stop once first solution found* (if (< row n)   (loop j below n       (when (is-safe row j n)               (setf (aref *chessboard* row j) 'board)               (backtracksearch (+ 1 row) n)               (setf (aref *chessboard* row j) 'nil))) (print-solution n))) 

i have tried use same implementation/logic in c++ solution of backtracking n-queens. advice on possible way forward helpful.

some feedback on code:

• it badly formatted, indent follows:

  (defun backtracksearch (row n)   (if (< row n)       (loop j below n             (when (is-safe row j n)                  (setf (aref *chessboard* row j) 'board)                  (backtracksearch (+ 1 row) n)                  (setf (aref *chessboard* row j) 'nil)))       (print-solution n))) 

• // comments in c++, use ; instead

• (dotimes (j n) (when ...)) sufficient.

some questions find solution:

• what happens when there no j satisfying is-safe?
• in particular, return value of backstracksearch?
• what return value when find solution?
• how determine if recursive call backtracksearch found solution?
• could use information avoid computing other solutions?