Upper panel: evolution of surface abundances of carbon, nitrogen and oxygen during the corrosive second dredge-up episode and the first thermal pulses of the TP-SAGB phase for the 8 M_⊙ model. Lower panel: evolution of the advance of the base of the convective envelope (grey shading) during the same interval of time. The lines have the same meaning as in Fig. 2. Letters a, b, c and d refer to the times for the corresponding panels of Fig. 4.