Spectral sequence of the nebular flux spectra of SN 2011fe. Epochs 7–10 show the evolution of forbidden [Fe] and [Co] lines. Epochs 7–10, colored as 7 (green), 8 (red), 9 (orange), and 10 (black), are normalized to the ∼4600 Å line. The progression of the [Co III] complex in the 5700–6700 Å wavelength range is apparent.