Temporal evolution of the (logarithm of the) hydrogen mass fraction X(¹H) at three different radii [temperatures] around the top, middle, and bottom of the helium-burning convection shell, respectively: r₁ = 8.8 × 10⁸ cm [T₁ ~ 0.8 × 10⁸ K] (dash-dotted), r₂ = 7.1 × 10⁸ cm [T₂ ~ 1.2 × 10⁸ K] (solid), and r₃ = 5.4 × 10⁸ cm [T₃ ~ 1.5 × 10⁸ K] (dotted). The upper panel (a) gives the results for the 3D model hifexp.3d.f, and the lower panel (b) for the 2D model hifexp2d.f, respectively.