The previous paragraphs discussed the case of overlapping
enqueue() by itself. This may happen when a
low-priority prologue is interrupted by a high-priority prologue
and both prologues request the invocation of their
associated epilogues. The other case to be handled is
that dequeue() is overlapped by enqueue(). This
may happen when a guarded section is going to be left
and, thus, pending epilogues are to be removed (employing
dequeue()) from the queue of the guard. The case that
needs not to be handled is when dequeue() interrupts itself
and enqueue(). This kind of overlapping would be
possible only if prologues decide to remove epilogues from
the queue—and this does not correspond to the pro/epilogue
concept of PURE.