anchor.diff:      ! Optical depth to travel
       real(dp) :: tau
   
  -    ! Used for the forced first interaction
  -    real(dp) :: tau_escape, weight
  -    type(photon) :: p_tmp
  -    logical :: killed
  -
       ! REMOVE
       real(dp) :: tau_achieved
   
  
positive.code:              ! If this is not the first interaction, and the user requested to
               ! use the MRW, do the MRW to get out of the optically thick region
  
               if(interactions > 1 .and. use_mrw) then
                  do mrw_steps=1,n_mrw_max
                     if(tau_inv_planck_to_closest_wall(p) > mrw_gamma) then
                        call grid_do_mrw(p)
  
negative.code:                 end if
               end if
  
               ! Sample a random optical depth and propagate that optical depth
               call random_exp(tau)
               call grid_integrate(p,tau,tau_achieved)
  
               if(p%reabsorbed) then
  