anchor.diff:    use performance, only : perf_header, perf_footer
   
     use counters, only : killed_photons_int
  -
  -  use forced_interaction, only : forced_interaction_baes16, WR99, BAES16
   
     implicit none
     save
  
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:                 end if
               end if
  
               call random_exp(tau)
  
               ! Propagate the optical depth sampled
               call grid_integrate(p,tau,tau_achieved)
  