Left: model spectrum computed using the standard single-line ALI approach for the radiative rates. The iteration is stable but does not converge. Instead, due to the mutual interaction of strong spectral lines of different ions, the model becomes stuck in a state where approximately 50% of the energy disappears. Right: spectrum from a model where all line opacities and emissivities are treated as purely coherent scattering. Here the flux is conserved, but too much energy is emitted in the blue part, in contradiction to the observations.