Program problem

[Lewis Creary via Users list for the GNU Emacs text editor <help-gnu-emacs@gnu.org>]

(It has been suggested to me by someone on the above mailing list thatthe following content would be appropriate for this list.)
My purpose here is to discuss a bug I've discovered in an emacs lispfunction, fill-rows, that I've written (shown below).  The main ideaof this function is to help solvers of Sudoku puzzles who want to keeptrack of the numbers they've already entered into a puzzle.  This willhelp them to backtrack when they change some of those numbers.  It'simportant to understand the documentation string of this function(included in the function definition) before reading the code.
The use case of interest involves this function call:   (fill-rows '(1 3 5)                  '("123456789" "987654321" "123498765")                   '((0 0 0 0 0 0 0 0 0)                    (0 0 0 0 0 0 0 0 0)                    (0 0 0 0 0 0 0 0 0)                    (0 0 0 0 0 0 0 0 0)                    (0 0 0 0 0 0 0 0 0)                    (0 0 0 0 0 0 0 0 0)                    (0 0 0 0 0 0 0 0 0)                    (0 0 0 0 0 0 0 0 0)                    (0 0 0 0 0 0 0 0 0)) )          
The desired value to be computed by this function call is:              '((1 2 3 4 5 6 7 8 9)                 (0 0 0 0 0 0 0 0 0)                (9 8 7 6 5 4 3 2 1)                (0 0 0 0 0 0 0 0 0)                (1 2 3 4 9 8 7 6 5)                (0 0 0 0 0 0 0 0 0)                (0 0 0 0 0 0 0 0 0)                (0 0 0 0 0 0 0 0 0)                (0 0 0 0 0 0 0 0 0))
The value (incorrectly) computed (this is the bug mentioned above) is:              '((0 0 0 0 0 0 0 0 0)                (9 0 0 0 0 0 0 0 0)                (0 0 0 0 0 0 0 0 0)                (0 0 0 0 0 0 0 0 0)                (0 0 0 0 0 0 0 0 0)                (0 0 0 0 0 0 0 0 0)                (0 0 0 0 0 0 0 0 0)                (0 0 0 0 0 0 0 0 0)                (0 0 0 0 0 0 0 0 0))
Here, then, is the function definition:
(defun fill-rows (row-nums change-strings pzl-form)  "This fn takes as arguments a list of row indices, a list of an equal   number, s, of 9-digit strings, and a puzzle form (i.e, an sexp of the form:                  ((n n n n n n n n n)                   (n n n n n n n n n)                   (n n n n n n n n n)                   (n n n n n n n n n)                   (n n n n n n n n n)                   (n n n n n n n n n)                   (n n n n n n n n n)                   (n n n n n n n n n)                   (n n n n n n n n n)), where n is a number between 0 and 9).   The fn returns as value t he result of filling in row m of the argument   puzzle form with numbers specified by the mth 9-digit string."  (let* ((row-index 0)           (row-num-index 0)           (row-str (nth row-index change-strings))           (row-num (nth row-num-index row-nums))           (change-nums nil)           (str-pos 0) )      (while (and (< row-index (length row-nums)) (< str-pos 9))             ;; (comment) cycle through the change-strings          (while (< str-pos 9)                 ;; cycle through row-str, contributing to change-list                   (setq change-nums (append change-nums                                                   (list (string-to-number                                                      (substring row-str str-pos (1+ str-pos)) )) )                        row-str (nth row-index change-strings) )              (setcar (car (nthcdr row-num pzl-form))                          (nth str-pos change-nums) )          (cl-incf str-pos) )       (cl-incf row-index)       (cl-incf row-num-index) )   pzl-form ) )
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I'll very much appreciate receiving any proposed bugfixes or other  suggestions you may have.
Thanks.
Lewis Creary