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How would I use a method belonging to OperationQueue by setting up a private serial DispatchQueue?

In 7. Operation Queues, under Waiting for completion, it says:

If you find yourself needing this method (OperationQueue waitUntilAllOperationsAreFinished), then you should set up a private serial DispatchQueue wherein you can safely call this blocking method.

My question is, how would I use waitUntilAllOperationsAreFinished – a method belonging to OperationQueue – by setting up a private serial DispatchQueue? Can I see a sample code?