anchor.diff:                  local-time:timestamp-day-of-week
                   zerop))))
   
  +(defun problem-20 ()
  +  ;; n! means n × (n − 1) × ... × 3 × 2 × 1
  +  ;;
  +  ;; For example, 10! = 10 × 9 × ... × 3 × 2 × 1 = 3628800,
  +  ;; and the sum of the digits in the number 10! is 3 + 6 + 2 + 8 + 8 + 0 + 0 = 27.
  +  ;;
  +  ;; Find the sum of the digits in the number 100!
  +  (sum (digits (factorial 100))))
  +
   
   ;;;; Tests --------------------------------------------------------------------
   (def-suite :euler)
  
positive.code: (test p17 (is (= 21124 (problem-17))))
  (test p18 (is (= 1074 (problem-18))))
  (test p19 (is (= 171 (problem-19))))
  
  
  ;; (run! :euler)
  
negative.code:   (iterate (for i :from 1 :to k)
             (multiply (/ (+ n 1 (- i))
                          i))))
  
  
  (defun factorial (n)
    (iterate (for i :from 1 :to n)
             (multiplying i)))
  
  
  ;;;; Problems -----------------------------------------------------------------
  