Useful for handling a certain kind of mutex, with some primitive
priority concept. There is a Master and several Slaves, Master
can block slaves from being able to acquire the lock, and take
the control itself once all slaves are done. Slaves share the same
lock, and can acquire every time they want, as many times as they want.
If you see the mutex object like a microphone, and more people talking
at a conference sharing it, where there's a boss that can hold his/her
employees from talking, you got it.
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