anchor.diff:  (test p52 (is (= 142857 (problem-52))))
   (test p53 (is (= 4075 (problem-53))))
   (test p54 (is (= 376 (problem-54))))
  -
  +(test p55 (is (= 249 (problem-55))))
   (test p56 (is (= 972 (problem-56))))
  +(test p57 (is (= 153 (problem-57))))
  +
   (test p74 (is (= 402 (problem-74))))
   (test p145 (is (= 608720 (problem-145))))
   
  
positive.code: 
  
  (defun digits-to-number (digits)
    (reduce (lambda (total digit)
              (+ (* total 10) digit))
            digits))
  
  
  (defun palindromep (n &optional (radix 10))
  
negative.code:   ;; There are 120 reversible numbers below one-thousand.
    ;;
    ;; How many reversible numbers are there below one-billion (10^9)?
    (flet ((reversiblep (n)
             (let ((reversed (reverse-integer n)))
               (values (unless (zerop (digit 0 n))
                         (every #'oddp (digits (+ n reversed))))
                       reversed))))
      (iterate
        ;; TODO: improve this one
        ;; (with limit = 1000000000) there are no 9-digit reversible numbers...
  