Const node iterator.
This is an "iterator to an iterator" - it iterates over nodes, and de-referencing it returns one of the tree's const iterators
| Type | Definition | Description | 
| size_type | container_base::size_type | Size type. | 
| Type | Definition | Description | 
| iterator_category | trivial_iterator_tag | Category. This tag identifies that the iterator has none of the STL's iterators' movement abilities. | 
| difference_type | void | Difference type. | 
| Type | Definition | Description | 
| value_type | container_base::const_iterator | Iterator's value type. | 
| reference | value_type | Iterator's reference type. | 
| const_reference | value_type | Iterator's const reference type. | 
| e_access_traits | trie::e_access_traits | Element access traits. | 
| const_e_iterator | typename e_access_traits::const_iterator | A key's element const iterator. | 
These are only defined if basic_tree::Node_Update is not null_trie_node_update
| Type | Definition | Description | 
| metadata_type | typename basic_tree::Node_Update::metadata_type | Metadata type. | 
| const_metadata_reference | 
typename Allocator::template rebind<
    metadata_type>::other::const_reference
 | Const metadata reference type. | 
| Method | Description | 
| inline const_node_iterator () | Default constructor. | 
| Method | Description | 
| 
inline std::pair<
    const_e_iterator,
    const_e_iterator>
  valid_prefix
  () const
 | Subtree valid prefix. Returns the common prefix range of all nodes in this node's subtree. | 
| inline const_reference operator* () const | Const access; returns the const iterator associated with the current leaf. Should be called only for leaf nodes. | 
These are only defined if basic_tree::Node_Update is not null_trie_node_update
| Method | Description | 
| inline const_metadata_reference get_metadata () const | Metadata access. | 
| Method | Description | 
| inline size_type num_children () const | Returns the number of children in the corresponding node. If the number of children is 0, then the corresponding node is a leaf; otherwise, it is not a leaf. | 
| const_node_iterator get_child (size_type i) const | Returns a const node iterator to the corresponding node's i-th child. | 
| Method | Description | 
| 
inline bool
  operator==
  (const const_node_iterator &other) const
 | Compares content to a different iterator object. | 
| 
inline bool
  operator!=
  (const const_node_iterator &other) const
 | Compares content (negatively) to a different iterator object. |