anchor.diff:                  end if
                end if
   
  -             ! If this is the first interaction and the user requested forced
  -             ! first interaction, we sample tau and modify the photon energy
  -             ! using a forced first interaction algorith - otherwise we sample
  -             ! tau the normal way.
  -
  -             if(interactions == 1 .and. forced_first_interaction) then
  -                select case(forced_first_interaction_algorithm)
  -                case(WR99)
  -                   ! Note that we don't do anything for the wr99 algorithm
  -                   ! because that algorithm only matters for small optical depths
  -                   ! but since we use the Lucy algorithm for the temperature,
  -                   ! in cases of low optical depths, all cells along the path
  -                   ! to escape will get contributions to the energy, so we
  -                   ! won't improve things by forcing a scattering.
  -                   call random_exp(tau)
  -                case(BAES16)
  -                   p_tmp = p
  -                   call grid_escape_tau(p_tmp, huge(1._dp), tau_escape, killed)
  -                   if(tau_escape > 1.e-10 .and. .not. killed) then
  -                      call forced_interaction_baes16(tau_escape, tau, weight)
  -                      p%energy = p%energy * weight
  -                   else
  -                     ! Fall back to normal sampling. In particular, if the
  -                     ! optical depth to escape is zero, we need to make sure
  -                     ! tau > 0 so the integration succeeds. In the test
  -                     ! photon was killed, we can't trust tau_escape and
  -                     ! therefore should just default back to this.
  -                     call random_exp(tau)
  -                   end if
  -                case default
  -                   call error("propagate", "Unknown forced first interaction algorithm")
  -                end select
  -             else
  -                call random_exp(tau)
  -             end if
  +             call random_exp(tau)
   
                ! Propagate the optical depth sampled
                call grid_integrate(p,tau,tau_achieved)
  
positive.code: 
    use grid_pda, only : solve_pda
  
    use settings, only : forced_first_interaction, &
         &               forced_first_interaction_algorithm, &
         &               kill_on_absorb, kill_on_scatter, &
         &               n_inter_max, &
         &               n_inter_max_warn, &
         &               mrw_gamma, &
  
negative.code:   use performance, only : perf_header, perf_footer
  
    use counters, only : killed_photons_int
  
    implicit none
    save
  