class Async::Semaphore

@returns The result of the block if invoked.
@yields {...} When the semaphore can be acquired.
If no block is provided, you must call release manually.
Acquire the semaphore, block if we are at the limit.

def acquire
	wait
	
	@count += 1
	
	return unless block_given?
	
	begin
		return yield
	ensure
		self.release
	end
end