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 letter-number (char)
    "Return the index of `char` in the alphabet (A being 1)."
    (1+ (- (char-code (char-upcase char)) (char-code #\A))))
  
  
  ;;;; Problems -----------------------------------------------------------------
  
negative.code: 
  
  
  (defun problem-52 ()
    ;; It can be seen that the number, 125874, and its double, 251748, contain
    ;; exactly the same digits, but in a different order.
    ;;
    ;; Find the smallest positive integer, x, such that 2x, 3x, 4x, 5x, and 6x,
    ;; contain the same digits.
    (iterate (for i :from 1)
             (for digits = (digits i))
             (finding i :such-that
                      (every (lambda (n)
                               (orderless-equal digits (digits (* n i))))
                             '(2 3 4 5 6)))))
  
  (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.
  