 Solution

The second episode starts only after the first headache, at 24.8; its row is absent from every risk set before that time. More generally, episode $k+1$ starts at the observed end of episode $k$. The left-open, right-closed interval convention assigns an event at a shared boundary only to the interval ending there. Thus the first event is counted once, and the second cannot be counted before the first. Preserving the episode entry times and patient history enforces the ordering; entering every recurrence as a new record starting at treatment time zero would not do so.