///It conforms to the graph interface documented under

   ///It conforms to the graph interface documented under

   ///the description of \ref GraphSkeleton with the exception that you cannot

   ///the description of \ref GraphSkeleton with the exception that you cannot

   ///add (or delete) edges. The usual edge iterators are exists, but they are

   ///add (or delete) edges. The usual edge iterators are exists, but they are

   ///always \ref INVALID.

   ///always \ref INVALID.

   ///\sa \ref GraphSkeleton

   ///\sa \ref GraphSkeleton

   class NodeSet {

   class NodeSet {

     //Nodes are double linked.

     //Nodes are double linked.

     //The free nodes are only single linked using the "next" field.

     //The free nodes are only single linked using the "next" field.

     struct NodeT 

     struct NodeT