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