and after the right subtree traversal,
- for leaf nodes: once.
The arguments passed to ACTION are:
- 1. the node; it can be casted to a 'const void * const *', i.e. into a
+ 1. the node; it can be cast to a 'const void * const *', i.e. into a
pointer to the key,
2. an indicator which visit of the node this is,
3. the level of the node in the tree (0 for the root). */