anchor.diff:      (pandigitalp 123 0 3) ; => nil
   
     "
  -  (equal (range start (1+ end))
  -         (sort (digits integer) #'<)))
  +  (equal (irange start end)
  +         (sort< (digits integer))))
   
   (defun pandigitals (&optional (start 1) (end 9))
     "Return a list of all `start` to `end` (inclusive) pandigital numbers."
  
positive.code:   ;; Find the difference between the sum of the squares of the first one hundred
    ;; natural numbers and the square of the sum.
    (flet ((sum-of-squares (to)
             (sum (range 1 (1+ to) :key #'square)))
           (square-of-sum (to)
             (square (sum (range 1 (1+ to))))))
      (abs (- (sum-of-squares 100) ; apparently it wants the absolute value
              (square-of-sum 100)))))
  
  
negative.code:   (let* ((limit 28123)
           (abundant-numbers
             (make-hash-set :initial-contents
                            (remove-if-not #'abundantp (irange 1 limit)))))
      (flet ((abundant-sum-p (n)
               (iterate (for a :in-hashset abundant-numbers)
                        (when (hset-contains-p abundant-numbers (- n a))
                          (return t)))))
        (sum (remove-if #'abundant-sum-p (irange 1 limit))))))
  
  (defun problem-24 ()
    ;; A permutation is an ordered arrangement of objects. For example, 3124 is
  