anchor.diff:    "
     (equal (range start (1+ end))
            (sort (digits integer) #'<)))
  +
  +(defun pandigitals (&optional (start 1) (end 9))
  +  "Return a list of all `start` to `end` (inclusive) pandigital numbers."
  +  (gathering
  +    (map-permutations (lambda (digits)
  +                        ;; 0-to-n pandigitals are annoying because we don't want
  +                        ;; to include those with a 0 first.
  +                        (unless (zerop (first digits))
  +                          (gather (digits-to-number digits))))
  +                      (range start (1+ end))
  +                      :copy nil)))
   
   
   (defun-inline digits< (n digits)
  
positive.code:            (triangle-word-p (word)
               (trianglep (word-value word))))
      (count-if #'triangle-word-p (parse-strings-file "data/42-words.txt"))))
  
  
  ;;;; Tests --------------------------------------------------------------------
  
negative.code: 
  (defun pandigitalp (integer &key (start 1) (end 9))
    "Return whether `integer` is `start` to `end` (inclusive) pandigital.
  
    Examples:
  
      (pandigitalp 123)     ; => nil
  