All Known Implementing Classes:
Nodes, NodeTree, RelativeStepFilter, SortedNodes, TreeList, Values, ValueStack, XMLFilter, XMLPrinter
An object that can be "fed" a TreePosition, and will do something with it.
That "something" can be printing it, copy it, filter it - or ignore it.
Consume node at current position.
The caller may invalidate or change the position after consume returns,
so if the consumer wants to save it, it needs to copy it.
Consume a single position pair.
This PositionConsumer may assume the sequence does no reference
management; i.e. that copyPos is trivial and releasePos is
a no-op. If that is not the case, use consume(TreePosition) instead.