anchor.diff:      (sort <> #'<)
       (elt <> (1- 1000000))))
   
  +(defun problem-25 ()
  +  ;; The Fibonacci sequence is defined by the recurrence relation:
  +  ;;
  +  ;;   Fn = Fn−1 + Fn−2, where F1 = 1 and F2 = 1.
  +  ;;
  +  ;; Hence the first 12 terms will be:
  +  ;;
  +  ;; F1 = 1
  +  ;; F2 = 1
  +  ;; F3 = 2
  +  ;; F4 = 3
  +  ;; F5 = 5
  +  ;; F6 = 8
  +  ;; F7 = 13
  +  ;; F8 = 21
  +  ;; F9 = 34
  +  ;; F10 = 55
  +  ;; F11 = 89
  +  ;; F12 = 144
  +  ;;
  +  ;; The 12th term, F12, is the first term to contain three digits.
  +  ;;
  +  ;; What is the index of the first term in the Fibonacci sequence to contain
  +  ;; 1000 digits?
  +  (iterate (for f :in-fibonacci t)
  +           (for i :from 1)
  +           (finding i :such-that (= 1000 (digits-length f)))))
  +
   
   ;;;; Tests --------------------------------------------------------------------
   (def-suite :euler)
  
positive.code: (defun collatz-length (n)
    (iterate (for i :in-collatz n)
             (counting t)))
  
  
  (defun binomial-coefficient (n k)
  
negative.code: (test p22 (is (= 871198282 (problem-22))))
  (test p23 (is (= 4179871 (problem-23))))
  (test p24 (is (= 2783915460 (problem-24))))
  (test p25 (is (= 4782 (problem-25))))
  
  
  ;; (run! :euler)
  