anchor.diff:          end if
   
           nstack = 0
  -        lo = 1
  +
  +        if (present(tail)) then
  +            lo = tail
  +        else
  +            lo = 1
  +        end if
           if (present(head)) then
               hi = head
           else
  
positive.code:         ! In/Out:
          !    list: 2D array of i0 integers.  Sorted on output.
          ! In:
          !    head (optional): sort list up to and including list(:,:head) and
          !        leave the rest of the array untouched.  Default: sort the
          !        entire array.
  
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.
  
  