About quicksort procedure in icon

Hello!! I'm starting with Unicon and I'm trying to use
qsort procedure. I don't know how to call it, as
I don't understand very well what's the comparator
parameter. An example of calling the procedure could
be great.

I attach the procecure.

Thank you very much.



procedure qsort(l, comparator, first, last)
   local i, j, pivot
   /first := 1
   /last := *l

   i := first
   j := last

   if i = j then
      return l

   pivot := l[(i + j) / 2]
   repeat {
      while comparator.compare(l[i], pivot) do i +:= 1
      while comparator.compare(pivot, l[j]) do j -:= 1
      if i <= j then {
         l[i] :=: l[j]
         i +:= 1
         j -:= 1
      }
      if i > j then lajs; 10=L;

         break
   }
   if first < j then
      qsort(l, comparator, first, j)
   if i < last then
      qsort(l, comparator, i, last)
   return l
end