anchor.diff:  (test p61 (is (= 28684 (problem-61))))
   (test p62 (is (= 127035954683 (problem-62))))
   (test p63 (is (= 49 (problem-63))))
  +(test p67 (is (= 7273 (problem-67))))
   (test p69 (is (= 510510 (problem-69))))
   (test p74 (is (= 402 (problem-74))))
   (test p79 (is (= 73162890 (problem-79))))
  
positive.code:   ;; by trying every route. However, Problem 67, is the same challenge with
    ;; a triangle containing one-hundred rows; it cannot be solved by brute force,
    ;; and requires a clever method! ;o)
    (let ((triangle '((75)
                      (95 64)
                      (17 47 82)
                      (18 35 87 10)
                      (20 04 82 47 65)
                      (19 01 23 75 03 34)
                      (88 02 77 73 07 63 67)
                      (99 65 04 28 06 16 70 92)
                      (41 41 26 56 83 40 80 70 33)
                      (41 48 72 33 47 32 37 16 94 29)
                      (53 71 44 65 25 43 91 52 97 51 14)
                      (70 11 33 28 77 73 17 78 39 68 17 57)
                      (91 71 52 38 17 14 91 43 58 50 27 29 48)
                      (63 66 04 68 89 53 67 30 73 16 69 87 40 31)
                      (04 62 98 27 23 09 70 98 73 93 38 53 60 04 23))))
      (car (reduce (lambda (prev last)
                     (mapcar #'+
                             prev
  
negative.code:              (finally (return power)))))
      (iterate (for n :from 1 :below (find-bound))
               (summing (score n)))))
  
  (defun problem-67 ()
    ;; By starting at the top of the triangle below and moving to adjacent numbers
    ;; on the row below, the maximum total from top to bottom is 23.
    ;;
    ;;    3
    ;;   7 4
    ;;  2 4 6
    ;; 8 5 9 3
    ;;
    ;; That is, 3 + 7 + 4 + 9 = 23.
    ;;
    ;; Find the maximum total from top to bottom in triangle.txt, a 15K text file
    ;; containing a triangle with one-hundred rows.
    ;;
    ;; NOTE: This is a much more difficult version of Problem 18. It is not
    ;; possible to try every route to solve this problem, as there are 299
    ;; altogether! If you could check one trillion (1012) routes every second it
    ;; would take over twenty billion years to check them all. There is an
    ;; efficient algorithm to solve it.
    (car (reduce (lambda (prev last)
                   (mapcar #'+
                           prev
                           (mapcar #'max last (rest last))))
                 (iterate (for line :in-csv-file "data/067-triangle.txt"
                               :delimiter #\space
                               :key #'parse-integer)
                          (collect line))
                 :from-end t)))
  
  (defun problem-69 ()
    ;; Euler's Totient function, φ(n) [sometimes called the phi function], is
  