numbers.cl

(defun digits-to-val (digits)
  (reduce (lambda (acc x)
            (+ (* 10 acc) x)) digits))

(defun val-to-digits (x)
    (if (< x 10)
       (list (mod x 10))
    (cons (mod x 10)
          (val-to-digits (floor x 10)))))


(defun remove-nth (n seq)
  (cond 
    ((null seq) seq)
    ((= 0 n) (cdr seq))
    (t (cons (car seq)
             (remove-nth (- n 1) (cdr seq))))))
 

(defun permutations (p)
  (if (null p)
      '(())
      (loop for i from 0 below (length p) append
            (let ((x (nth i p)))
              (mapcar (lambda (tail)
                        (cons x tail))
                      (permutations (remove-nth i p)))))))