;;; Gray code
;;; Make it with lists first, then make string from list
;;; Way of making it: for n = 0 it is the empty list
;;; For larger n, add 0 to code for n-1, and add 1 to reverse

(defun gray (n)
  (mapcar #'list-to-string (gray-list n))
  )

(defun list-to-string (lista)
  (let ((a (make-string (length lista) :initial-element #\0)))
    (dotimes (i (length lista) a)
      (setf (aref a i) (elt lista i))
      )
    )
  )

(defun gray-list (n)
  (if (= n 0)
      '(())
    (let ((prev (gray-list (1- n))))
      (append (add #\0 prev)
	      (add #\1 (reverse prev))
	      )
      )
    )
  )

;;;  Adding a digit to a previous code word

(defun add (digit word)
  (mapcar #'(lambda (x) (cons digit x)) word)
  )