pub fn propose<G, Tr, K, F, P, V>(
    prefixes: &Collection<G, P, Tr::Diff>,
    arrangement: Arranged<G, Tr>,
    key_selector: F,
) -> Collection<G, (P, V), Tr::Diff>
where
    G: Scope<Timestamp=Tr::Time>,
    Tr: TraceReader+Clone+'static,
    for<'a> Tr::Key<'a> : IntoOwned<'a, Owned = K>,
    K: Hashable + Default + Ord + 'static,
    Tr::Diff: Monoid+Multiply<Output = Tr::Diff>+ExchangeData,
    for<'a> Tr::Diff : Semigroup<Tr::DiffGat<'a>>,
    F: Fn(&P)->K+Clone+'static,
    P: ExchangeData,
    V: Clone + 'static,
    for<'a> Tr::Val<'a> : IntoOwned<'a, Owned = V>,
{
    crate::operators::lookup_map(
        prefixes,
        arrangement,
        move |p: &P, k: &mut K | { *k = key_selector(p); },
        move |prefix, diff, value, sum| ((prefix.clone(), value.into_owned()), diff.clone().multiply(sum)),
        Default::default(),
        Default::default(),
        Default::default(),
    )
}
Proposes extensions to a prefix stream.

This method takes a collection `prefixes` and an arrangement `arrangement` and for each
update in the collection joins with the accumulated arranged records at a time less or
equal to that of the update. Note that this is not a join by itself, but can be used to
create a join if the `prefixes` collection is also arranged and responds to changes that
`arrangement` undergoes. More complicated patterns are also appropriate, as in the case
of delta queries.
dogsdogsdogs/src/operators/propose.rs