that while SNe 2008D [@Modjaz09] and 1993J [@Matheson00] have comparable \] strength, this feature is quite strong in the broad-lined Type Ic (Ic-BL) SNe 2006aj [@Mazzali07:06aj] and 2003jd [@Valenti08].

Again comparing [SN 2001ig]{} to the SNe that appeared in @Foley03 [Table 6], it has a Mg/O ratio comparable to that of SNe 1994I and 2002ap, but larger than that of SNe 1998bw and 1985F. In addition, the Mg/O ratio of [SN 2001ig]{} is similar to that of SNe 2008D, 2006aj, and 2003jd (which in fact has quite a large ratio), but significantly greater than that of SN 1993J. This confirms the claim of @Filippenko02 that [SN 2001ig]{} had the strongest nebular O/Mg emission-line ratio of any SN observed at that time. Table \[t:lines\] also seems to indicate that it is not uncommon for core-collapse SNe to have relatively large Mg/O ratios, yet what distinguishes between the SNe that do or do not show this is unclear.

Spectral Models {#s:model}
===============

By several months after the explosion of a SN, the densities in the ejecta are sufficiently low from expansion that the opacity drops below unity and the gas obeys nebular physics. At this time it is heated by radioactive decay 