> Here, removing a thread
> from a wait queue other than the one from which it was signalled is
> O(waiting threads).

To be fair, You will probably never have more than a few hundred/thousand threads on a process. Usually you'll work under a few dozen threads.

To reduce the complexity on average, you could use a set, but `notify` no longer will be able to follow insertion order.

I was hoping it could be done later...