anchor.diff:  (test p14 (is (= 837799 (problem-14))))
   (test p15 (is (= 137846528820 (problem-15))))
   (test p16 (is (= 1366 (problem-16))))
  +(test p17 (is (= 21124 (problem-17))))
   
   
   ;; (run! :euler)
  
positive.code:   ;; What is the sum of the digits of the number 2^1000?
    (sum (digits (expt 2 1000))))
  
  
  ;;;; Tests --------------------------------------------------------------------
  (def-suite :euler)
  
negative.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))
  