Default subscription implementations.
A mutable collection of subscriptions, which is itself a subscription.
When unsubscribed from, all containing subscriptions are unsubscribed.
Subsequently added subscriptions are automatically unsubscribed.
Subscription composed of several subscriptions.
When unsubscribed, all the component subscriptions get unsubscribed.
Forwards unsubscribe to another subscription.
Returns a subscription that runs the specified action when unsubscribed.
The action **will not be** executed more than once.
Does not unsubscribe from anything.