Function interchange::envelopes::combine_at_timestamp

pub fn combine_at_timestamp<G: Scope>(
    batches: Stream<G, Rc<OrdValBatch<Option<Row>, Row, G::Timestamp, Diff>>>
) -> Collection<G, (Option<Row>, Vec<DiffPair<Row>>), Diff> where
    G::Timestamp: Lattice + Copy, 

Given a stream of batches, produce a stream of (vectors of) DiffPairs, in timestamp order.