Fwd: Extending Collector to handle a post-transform

[Ali Lahijani]

> public interface Collector<T, R> {
>     <X> Tuple<T, X, R> asTuple();

That implies a universal quantification on the variable X, i.e an
implementation of Collector should support any possible values that a
caller might supply for X.

I believe the correct signature to be
     Tuple<T, ?, R> asTuple();

that is an existential quantification on the anonymous type variable.
The Collector implementation decides what to put in the type variable.

That's also what you expect as a stand-in for

>    interface Collector<T,R> extends BaseCollector<T,?,R> { }

Ali