To execute, if you have quicklisp:

 (ql:quickload "cl-utilities")
 (ql:quickload "sudoku")

And now you can use it:

 (in-package #:sudoku)
 (print-sudoku (read-sudoku "sudoku.ss"))
 ->
  *-----------*
  |...|...|...|
  |2..|8.4|9.1|
  |...|1.6|32.|
  |---+---+---|
  |...|..5|.4.|
  |8..|423|..6|
  |.3.|9..|...|
  |---+---+---|
  |.63|7.9|...|
  |4.9|5.2|..8|
  |...|...|...|
  *-----------*

 (let ((s (read-sudoku "sudoku.ss"))) (backtracker-solver s) (print-sudoku s))
 ->
 *-----------*
 |341|297|685|
 |256|834|971|
 |987|156|324|
 |---+---+---|
 |192|675|843|
 |875|423|196|
 |634|918|257|
 |---+---+---|
 |563|789|412|
 |419|562|738|
 |728|341|569|
 *-----------*