anchor.diff:          use bit_utils, only: operator(.bitstrgt.)
   
           integer(i0), intent(inout) :: list(:,:)
  -        integer, intent(in), optional :: head, nsort
  +        integer, intent(in), optional ::  head, nsort, tail
   
           ! Threshold.  When a sublist gets to this length, switch to using
           ! insertion sort to sort the sublist.
  
positive.code: 
  !--- In-place quicksort.  Uses the sample code in Numerical Recipies as a base.  ---
  
      pure subroutine qsort_i0_list(list, head, nsort)
  
          ! Sort a 2D array of i0 integers.
  
  
negative.code:         ! 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.
  