Cancelable streams

[Brian Goetz]

> In essence I think you want to insert a "gate" operation into the pipeline - while the gate is open elements can be read from the stream, but when it is closed they can not. The question then becomes how you control the gate, either from within the pipeline (predicate based) or externally (time-trigerred callback).

Yes, that is exactly what I am suggesting.  Plus, the ability to query the gate to determine whether the pipeline finished because it was out of elements, or because the gate was manually closed.