In which lines of the code for
RB-DELETE-FIXUPmight 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