make-number-set.el

(defun dmaus:make-number-set (list)
    (when list
      (let ((first (car list))
            (index (car list)))
        (while (and (cadr list) (= (incf index) (cadr list)))
          (setq list (cdr list)))
        (cond
         ((= first (car list))
          (cons (format "%d" first) (dmaus:make-number-set (cdr list))))
         ((= (1+ first) (decf index))
          (cons (format "%d,%d" first index) (dmaus:make-number-set (cdr list))))
         (t
          (cons (format "%d-%d" first (car list)) (dmaus:make-number-set (cdr list))))))))