  The neutrino flux close to a supernova core contributes substantially to
neutrino refraction so that flavor oscillations become a nonlinear phenomenon.
One unexpected consequence is efficient flavor transformation for
anti-neutrinos in a region where only neutrinos encounter an MSW resonance or
vice versa. Contrary to previous studies we find that in the neutrino-driven
wind the electron fraction Y_e always stays below 0.5, corresponding to a
neutron-rich environment as required by r-process nucleosynthesis. The relevant
range of masses and mixing angles includes the region indicated by LSND, but
not the atmospheric or solar oscillation parameters.
