A format for expressing an ordered list of integers is to use a comma separated list of either

 individual integers
 Or a range of integers denoted by the starting integer separated from the end integer in the range by a dash, '-'. (The range includes all integers in the interval including both endpoints)
 The range syntax is to be used only for, and for every range that expands to more than two values.
Example

The list of integers:

 -6, -3, -2, -1, 0, 1, 3, 4, 5, 7, 8, 9, 10, 11, 14, 15, 17, 18, 19, 20
Is accurately expressed by the range expression:

 -6,-3-1,3-5,7-11,14,15,17-20
(And vice-versa).



Task

Expand the range description:

  -6,-3--1,3-5,7-11,14,15,17-20 

Note that the second element above,
is the range from minus 3 to minus 1. 



Related task

   Range extraction


(defun expand-ranges (string)
  (loop
     with prevnum = nil
     for idx = 0 then (1+ nextidx)
     for (number nextidx) = (multiple-value-list
                             (parse-integer string
                                            :start idx :junk-allowed t))
     append (cond
              (prevnum
               (prog1
                   (loop for i from prevnum to number
                      collect i)
                 (setf prevnum nil)))
              ((and (< nextidx (length string))
                    (char= (aref string nextidx) #\-))
               (setf prevnum number)
               nil)
              (t
               (list number)))
     while (< nextidx (length string))))
 
CL-USER> (expand-ranges "-6,-3--1,3-5,7-11,14,15,17-20")
(-6 -3 -2 -1 3 4 5 7 8 9 10 11 14 15 17 18 19 20)