ExtendedNodeVisitor

Template for extended node visitors. While the default node visitor of the type NodeT => U passes solely the inner node being visited, extended node visitors pass the following traversal state information:

1. the inner node currently visited as with a standard node visitor
2. the number of nodes visited so far and
3. the current depth in terms of the underlying algorithm and
4. a reference to a specific informer that may be pattern matched to collect even further data specific to the implementation.