bool operator<(const Node& node) const {return id < node.id;}

       bool operator<(const Node& node) const {return id < node.id;}

     };

     };

     class UEdge {

     class UEdge {

       friend class ListUGraphBase;

       friend class ListUGraphBase;

     protected:

     protected:

       int id;

       int id;

       explicit UEdge(int pid) { id = pid;}

       explicit UEdge(int pid) { id = pid;}