anchor.diff:          ! In/Out:
           !    list: 2D array of i0 integers.  Sorted on output.
           ! In:
  +        !    tail (optional): sort list beginning from tail.
           !    head (optional): sort list up to and including list(:,:head) and
           !        leave the rest of the array untouched.  Default: sort the
           !        entire array.
  
positive.code:         end if
  
          nstack = 0
          lo = 1
          if (present(head)) then
              hi = head
          else
  
negative.code: 
  !--- In-place quicksort.  Uses the sample code in Numerical Recipies as a base.  ---
  
      pure subroutine qsort_i0_list(list, head, nsort, tail)
  
          ! Sort a 2D array of i0 integers.
  
  