Node iterator.
This is an iterator to an iterator 
 - it
    iterates over nodes, and de-referencing it returns one of the
    tree's iterators
| Class | Derivation Type | 
| tree::const_node_iterator | public | 
| Method | Description | 
| inline node_iterator () | Default constructor. | 
| Method | Description | 
| inline container_base::iterator operator* () const | Access. | 
| Method | Description | 
| 
inline node_iterator
  get_l_child
  () const
 | Returns the node iterator associated with the left node. | 
| 
inline node_iterator
  get_r_child
  () const
 | Returns the node iterator associated with the right node. |