anchor.diff:  
     end subroutine grid_escape_tau
   
  -  subroutine grid_escape_column_density(p,tmax,column_density,killed)
  +  subroutine grid_escape_column_density(p_orig,tmax,column_density,killed)
   
       implicit none
   
  -    type(photon),value :: p
  +    ! This should work with recent compilers but not widespread yet
  +    ! type(photon),value :: p
  +    type(photon),intent(in) :: p_orig
  +    type(photon) :: p
  +
       real(dp),intent(in) :: tmax
       real(dp),intent(out) :: column_density(:)
       logical,intent(out) :: killed
  
positive.code: 
    end subroutine grid_integrate_noenergy
  
    subroutine grid_escape_tau(p,tmax,tau,killed)
  
      implicit none
  
      type(photon),value :: p
      real(dp),intent(in) :: tmax
      real(dp),intent(out) :: tau
      logical,intent(out) :: killed
  
negative.code:     real(dp) :: xi
      integer :: source_id
  
      p = p_orig
  
      killed = .false.
  
      radial = (p%r .dot. p%v) > 0.
  