wants to notify/return data to the client
the Sink interface to ActiveObject
Lister, calls _sink->DataReady
Acumulator implements Sinkinterface (see previous
We are now looking from the
perspective of the Lister. It pokes the Accumulator by calling its DataReady
method. From the point of view of the Lister, the Accumulator is nothing but
a DataSink (an interface).