anchor.diff:      implicit none
       type(photon),intent(in) :: p
       real(dp) :: d1,d2,d3,d4,d5,d6
  -    ! Is this the second nearest neighbor?
  +    call error("distance_to_closest_wall", "not implemented for Voronoi grid")
  +    ! TODO: is this the second nearest neighbor?
     end function distance_to_closest_wall
   
     subroutine find_wall(p,radial,tmin,id_min)
  
positive.code:     case(3)
         cell_width = 0.  ! TODO
      end select
    end function cell_width
  
    real(dp) function cell_area(cell, iface)
      implicit none
      type(grid_cell),intent(in) :: cell
      integer,intent(in) :: iface
      select case(iface)
      case(1,2)
         cell_area = 0.  ! TODO
  
negative.code:     case(5,6)
         cell_area = 0.  ! TODO
      end select
  
    end function cell_area
  
    ! Standard Routines
  