anchor.diff:            enddo
          enddo
   
  +       ! finally, sum up all the jumps
   
          ! density
          Q_l%data(i+1,iqdens) = 0.0_dp_t
  
positive.code:     !  q(xi) = qminus + xi*(qplus - qminux + q6 * (1-xi) )
      !
      ! with xi = (x - xl)/dx, where xl is the interface of the left
      ! edge of the cell.  (Note that the Castro paper and Colella &
      ! Sekora (2008), seem to have a typo in the definition of xi --
      ! they write it with respect to the zone center, not left edge.
  
      ! Limit (C&W Eq. 1.10).  Here the loop is over cells, and
      ! considers the values on either side of the center of the cell
      ! (Qminus and Qplus).
      do n = 1, nprim
         do i = U%grid%lo-1, U%grid%hi+1
  
  
negative.code:     !         -+------+------+------+------+------+------+--   
      !          |     i-1     |      i      |     i+1     |   
      !                       * *           * 
      !                   q_l,i  q_r,i   q_l,i+1 
      !           
      ! q_l,i+1 are computed using the information in zone i.
      !
  
      do i = U%grid%lo-1, U%grid%hi+1
  