Example calculation for an interaction at a very late time in the nebular phase (black). The ejecta is limited to a 1 M_⊙ dense shell moving at 10 000 km s⁻¹ and powered at a rate of 3 × 10⁴¹ erg s⁻¹. A uniform and strong clumping corresponding to a volume filling factor of 1% was used to compensate for the spatial spread of the dense shell (set to ∼10% of the local radius). This spectrum is reminiscent of the observations of SN 1993J at a similar epoch (red; Matheson et al. 2000). The observed spectrum was corrected for redshift and reddening, and then normalized at 4800 Å to match the model spectrum.