anchor.diff:  
     !-------------------------------------------------------------------------
     !Subroutine in order to compute variables H, rho, cs, nu, Q_plus, Q_minus,
  -  !K_ff, K_e, tau_eff, E_ff,Fz given T, Sigma and Omega
  +  !K_ff, K_e, tau_eff, E_ff,Fz given T, Sigma and k, the position
     !------------------------------------------------------------------------
  -  subroutine variables(T, Sigma, Omega, f, optical_depth, tau_eff)
  -    implicit none
  -
  -    real(kind = x_precision),intent(in)  :: T,Sigma,Omega
  +  subroutine variables(k, T, Sigma, f, optical_depth, tau_eff)
  +    implicit none
  +
  +    real(kind = x_precision),intent(in)  :: T,Sigma
       integer, intent(in)                  :: optical_depth
  +    integer, intent(in)                  :: k
       real(kind = x_precision),intent(out) :: f,tau_eff
       real(kind = x_precision)             :: coeff_a,coeff_b,coeff_c
       real(kind = x_precision)             :: delta
   
  +    real(kind = x_precision)             :: Omega
       real(kind = x_precision)             :: H
       real(kind = x_precision)             :: rho
       real(kind = x_precision)             :: cs
  
positive.code:         Smax          = S_max
  
          ! S found with the dichotomy approach
          sigma_t_thin(i)  = dichotomy(Smin, Smax, eps, temperature(i), omega, optical_depth)
  
        enddo
  
  
negative.code:     !------------------------------------------------------------------------
  
      do k = 1, n_cell
        ! For each point of the S curve
        do i = 1, nb_it
          ! Optical thick case (tau >= 1)
  