  Here we revisit line identifications of type I supernovae and highlight trace
amounts of unburned hydrogen as an important free parameter for the composition
of the progenitor. Most 1-dimensional stripped-envelope models of supernovae
indicate that observed features near 6000-6400 Ang in type I spectra are due to
more than Si II 6355. However, while an interpretation of conspicuous Si II
6355 can approximate 6150 Ang absorption features for all type Ia supernovae
during the first month of free expansion, similar identifications applied to
6250 Ang features of type Ib and Ic supernovae have not been as successful.
When the corresponding synthetic spectra are compared to high quality
time-series observations, the computed spectra are frequently too blue in
wavelength. Some improvement can be achieved with Fe II lines that contribute
red-ward of 6150 Ang, however the computed spectra either remain too blue, or
the spectrum only reaches fair agreement when the rise-time to peak brightness
of the model conflicts with observations by a factor of two. This degree of
disagreement brings into question the proposed explosion scenario. Similarly, a
detection of strong Si II 6355 in the spectra of broad-lined Ic and
super-luminous events of type I/R is less convincing despite numerous model
spectra used to show otherwise. Alternatively, we suggest 6000-6400 Ang
features are possibly influenced by either trace amounts of hydrogen, or
blue-shifted absorption and emission in Halpha, the latter being an effect
which is frequently observed in the spectra of hydrogen-rich, type II
supernovae.
