Solid carbon-oxygen white dwarf cores have been shown to be likely initial configurations for collapse to neutron star densities. Solidification seems to entail carbon/oxygen separation, with oxygen settling at the star's center and carbon being confined to more external, lower-density layers. Electron captures on 16O are then the triggering mechanism for collapse. The authors elucidate the outcome of the complete reaction network started by those captures and derive simple expressions for accurately describing the approach to nuclear statistical equilibrium along this hitherto neglected evolutionary path