///\todo undocumented

       ///\todo undocumented

       ///

       ///

       typedef True UndirTag;

       typedef True UndirTag;

       /// or in other words, the trivial node iterator.

       /// or in other words, the trivial node iterator.

       /// This is the base type of each node iterator,

       /// This is the base type of each node iterator,

       /// thus each kind of node iterator converts to this.

       /// thus each kind of node iterator converts to this.

       /// More precisely each kind of node iterator should be inherited 

       /// More precisely each kind of node iterator should be inherited 

       /// from the trivial node iterator.

       /// from the trivial node iterator.

       class Node {

       class Node {