sudoku>(cl-format true "~{~{~a~^ ~}~%~}"
 (solve [[3 9 4 0 0 2 6 7 0]
         [0 0 0 3 0 0 4 0 0]
         [5 0 0 6 9 0 0 2 0]
         [0 4 5 0 0 0 9 0 0]
         [6 0 0 0 0 0 0 0 7]
         [0 0 7 0 0 0 5 8 0]
         [0 1 0 0 6 7 0 0 8]
         [0 0 9 0 0 8 0 0 0]
         [0 2 6 4 0 0 7 3 5]])
3  9  4  8  5  2  6  7  1
2  6  8  3  7  1  4  5  9
5  7  1  6  9  4  8  2  3
1  4  5  7  8  3  9  6  2
6  8  2  9  4  5  3  1  7
9  3  7  1  2  6  5  8  4
4  1  3  5  6  7  2  9  8
7  5  9  2  3  8  1  4  6
8  2  6  4  1  9  7  3  5

nil