  The prompt nu_e burst from a core-collapse supernova (SN) is subject to both
matter-induced flavor conversions and strong neutrino-neutrino refractive
effects. For the lowest-mass progenitors, leading to O-Ne-Mg core SNe, the
matter density profile can be so shallow that the usual MSW matter effects
occur within the dense-neutrino region close to the neutrino sphere. In this
case a ``split'' occurs in the emerging spectrum, i.e., the nu_e flavor
survival probability shows a step-like feature. We explain this feature
analytically as a ``MSW prepared spectral split.'' In a three-flavor treatment,
the step-like feature actually consists of two narrowly spaced splits. They are
determined by two combinations of flavor-lepton numbers that are conserved
under collective oscillations.
