@SuppressWarnings("unchecked")
    public void completeLateData() {
        incomingData.addLast(new StringData(incomingData.getOwner().request(),
                                            "A late hello, world!"));
    }
Called by some other data producing thread, later
container-core/src/test/java/com/yahoo/processing/test/documentation/AsyncDataProducer.java