This example is similar in purpose to the partial-unmarshalling sample: it allows
access to single purchase order objects before the whole input document is
unmarshalled. This reduces the memory consumption and speeds up the turn around
time.

While the partial-unmarshalling sample feeds the input document piecemeal to the
unmarshaller this sample works notification style. Instead of storing all purchase
orders in the parent object, a listener is called for each child. The listener
logic and installation can be found in the classes primer.PurchaseOrders and Main.
