Figure out the FIFO principle for command like key sequence

Command like key sequence doesn't necessarily satisify the
'a down a up b down b up' pattern.

No, if we type fast enough, it maybe like this:
'a down b down a up b up'

That's FIFO, see key.TriggerPool.notified():
when some key is pressed, we put its id into a queue,
then when some key is released, we take its id and compare
what we got from the queue, they will be identical.