Left: unmixed chemical composition versus Lagrangian mass in the s15A ejecta model of Woosley & Heger (2007) at 345 d (the original mass fraction is shown for ⁵⁶Ni), arising from a 15 M_⊙ progenitor star on the main sequence, and having an ejecta kinetic energy of 1.2 × 10⁵¹ erg, a total ejecta mass of 10.96 M_⊙, and 0.083 M_⊙ of ⁵⁶Ni initially. To properly identify the eight shells, we also show the profiles for C, N, Mg, and Ca. Right: composition profile obtained with the new mixing approach presented in this Letter, using M_sh = 5.36 M_⊙, in which only macroscopic mixing was applied by splitting and shuffling ejecta shells identified in the unmixed model shown on the left. In both panels (and in Fig. 2), the origin of the x-axis is at 1.83 M_⊙, which corresponds to the mass of the compact remnant.