A subscription to a certain kind of event, event processing or computation.
Calling unsubscribe on the subscription causes the events to no longer be
propagated to this subscription, or some computation to cease.
Unsubscribing is idempotent -- calling unsubscribe second time does nothing.
Stops event propagation on the corresponding event stream.
Returns a new subscription that unsubscribes this, and then executes an action.
Returns a new subscription that unsubscribes this subscription, and then another.