Function merge 

pub fn merge<K, T, D, I>(subscriptions: I) -> SelectAll<Labelled<K, T, D>>
where
    K: Clone + Unpin,
    T: Send + 'static,
    D: Send + 'static,
    I: IntoIterator<Item = (K, Subscription<T, D>)>,

r[impl jetstream.subscription.surface.composition] Merge many subscriptions without losing which one ended, or with what. Fan-in — use case 2 — and a terminal value — use case 4 — are each easy alone; this is the combination the document listed separately and never checked.