Package subscriber provides a subscription tree implementation. The New function creates a subscription and returns a Subscriber interface. It allows a publisher and subscribing client to communicate about a subscription. The publisher can see whether the client is still subscribed by polling the Closed method. The client can control the subscription by either calling Unsubscribe itself or when it has defered subscription management to other code call Wait to wait for other code to cancel the subscription. Once the client has called Unsubscribe on the subscription, that subscription will then call Unsubscribe on all of its children created through the Add method. After that it will become lifeless, meaning no events will ever be triggered by it anymore. In case the publisher decides it wants to stop publishing, it MUST NEVER call Unsubscribe itself on a subscriber, but should indicate the desire through other (out-of-band) means to the subscribing client who must then call Unsubscribe itself. The client that receives a Subcriber interface can call Wait to wait for the subscription to be canceled. That may be done by the code that e.g. created the subscription in responese to some external event. The client can also keep a reference to the subscription and call Unsubscribe itself to indicate to the publisher it is no longer interested in receiving data associated with the subscription. The implementation is designed to be used from concurrently running goroutines. It uses WaitGroups, Mutexes and atomic reference counting.