anchor.diff:    ;; 4, 5, 6, 7, 8 and 9?
     (-<> "0123456789"
       (gathering-vector (:size (factorial (length <>)))
  -      (map-permutations #'gather <>))
  -    (map-into <> #'parse-integer <>)
  +      (map-permutations (compose #'gather #'parse-integer) <>
  +                        :copy nil))
       (sort <> #'<)
       (elt <> (1- 1000000))))
   
  
positive.code: (test p29 (is (= 9183 (problem-29))))
  (test p30 (is (= 443839 (problem-30))))
  (test p31 (is (= 73682 (problem-31))))
  
  
  ;; (run! :euler)
  
negative.code:   (if (zerop n)
      1
      (values (1+ (truncate (log (abs n) radix))))))
  
  
  (defun digits-to-number (digits)
    (iterate (with result = 0)
             (for d :in-whatever digits)
             (mulf result 10)
             (incf result d)
             (finally (return result))))
  
  
  (defun definitely-palindrome-p (n)
  