anchor.diff:  (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)
  
positive.code:     (sort <> #'<)
      (elt <> (1- 1000000))))
  
  
  ;;;; Tests --------------------------------------------------------------------
  (def-suite :euler)
  
negative.code: (defun collatz-length (n)
    (iterate (for i :in-collatz n)
             (counting t)))
  
  
  (defmacro-driver (FOR var IN-FIBONACCI _)
    (declare (ignore _))
    (with-gensyms (a b)
      (let ((kwd (if generate 'generate 'for)))
        `(progn
           (with ,a = 0)
           (with ,b = 1)
           (,kwd ,var :next (prog1 ,b
                              (psetf ,a ,b
                                     ,b (+ ,a ,b))))))))
  
  (defun fibonacci (n)
    "Return the first `n` Fibonacci numbers as a fresh list."
    (iterate (repeat n)
             (for i :in-fibonacci t)
             (collect i)))
  
  
  (defun binomial-coefficient (n k)
  