pub fn validate<G, K, V, Tr, F, P>(
    extensions: &Collection<G, (P, V), 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, V)>,
    for<'a> Tr::Diff : Semigroup<Tr::DiffGat<'a>>,
    K: Ord+Hash+Clone+Default + 'static,
    V: ExchangeData+Hash+Default,
    Tr::Diff: Monoid+Multiply<Output = Tr::Diff>+ExchangeData,
    F: Fn(&P)->K+Clone+'static,
    P: ExchangeData,
{
    crate::operators::lookup_map(
        extensions,
        arrangement,
        move |(pre,val),key| { *key = (key_selector(pre), val.clone()); },
        |(pre,val),r,_,_| ((pre.clone(), val.clone()), r.clone()),
        Default::default(),
        Default::default(),
        Default::default(),
    )
}
Proposes extensions to a stream of prefixes.

This method takes a stream of prefixes and for each determines a
key with `key_selector` and then proposes all pair af the prefix
and values associated with the key in `arrangement`.
dogsdogsdogs/src/operators/validate.rs