sources = new ArrayList<ReleasableIterator<EntityContainer>>();
sources.add(new UpcastIterator<EntityContainer, BoundContainer>(
new BoundContainerIterator(new ReleasableAdaptorForIterator<Bound>(bounds.iterator()))));
sources.add(new UpcastIterator<EntityContainer, NodeContainer>(
new NodeContainerIterator(nodeDao.iterate())));
sources.add(new UpcastIterator<EntityContainer, WayContainer>(
new WayContainerIterator(wayDao.iterate())));
sources.add(new UpcastIterator<EntityContainer, RelationContainer>(
new RelationContainerIterator(relationDao.iterate())));