  We show that the bump in the electron number density profile at the base of
the hydrogen envelope in O-Ne-Mg core-collapse supernovae causes an interesting
interplay between neutrino-electron and neutrino-neutrino forward scattering
effects in the flavor evolution of low-energy nu_e in the neutronization burst.
The bump allows a significant fraction of the low-energy nu_e to survive by
rendering their flavor evolution nonadiabatic. Increasing the luminosity of the
neutronization burst shifts the bump-affected nu_e to lower energy with reduced
survival probability. Similarly, lowering the luminosity shifts the
bump-affected neutrinos to higher energies. While these low energy neutrinos
lie near the edge of detectability, the population of bump-affected neutrinos
has direct influence on the spectral swap formation in the neutrino signal at
higher energies.
