Exercise 13.4.4

In which lines of the code for RB-DELETE-FIXUP might we examine or modify the sentinel $T.nil$?

$x$ can initially be the sentinel, and all the checks and modifications against it (lines 2, 3, 6, 7, 8, 11, 16, 17, 18, 20) examine the sentinel. It happens to have the right pointers, because RB-DELETE has set them up before calling RB-DELETE-FIXUP.