anchor.diff:  (test p47 (is (= 134043 (problem-47))))
   (test p48 (is (= 9110846700 (problem-48))))
   
  +(test p52 (is (= 142857 (problem-52))))
   (test p56 (is (= 972 (problem-56))))
   (test p74 (is (= 402 (problem-74))))
   
  
positive.code: 
  
  
  (defun problem-56 ()
    ;; A googol (10^100) is a massive number: one followed by one-hundred zeros;
    ;; 100^100 is almost unimaginably large: one followed by two-hundred zeros.
  
negative.code: (defun letter-number (char)
    "Return the index of `char` in the alphabet (A being 1)."
    (1+ (- (char-code (char-upcase char)) (char-code #\A))))
  
  
  (defun set-equal (list1 list2 &rest args)
    (null (apply #'set-exclusive-or list1 list2 args)))
  
  (defun orderless-equal (list1 list2 &key (sort-predicate #'<))
    (equal (sort (copy-seq list1) sort-predicate)
           (sort (copy-seq list2) sort-predicate)))
  
  
  ;;;; Problems -----------------------------------------------------------------
  