anchor.diff:  (test p18 (is (= 1074 (problem-18))))
   (test p19 (is (= 171 (problem-19))))
   (test p20 (is (= 648 (problem-20))))
  +(test p21 (is (= 31626 (problem-21))))
   
   
   ;; (run! :euler)
  
positive.code:           (not (dividesp n 11))) nil)
      (t (definitely-palindrome-p n))))
  
  (defun sum (sequence &key key)
    (iterate (for n :in-whatever sequence)
             (sum (if key
                    (funcall key n)
                    n))))
  
  (defun divisors (n)
    (sort (iterate (for i :from 1 :to (sqrt n))
  
negative.code:                    (collect i)
                     (collect (/ n i))))
          #'<))
  
  (defun proper-divisors (n)
    (remove n (divisors n)))
  
  (defun count-divisors (n)
    (* 2 (iterate (for i :from 1 :to (sqrt n))
  