///restore() function.

     ///restore() function.

     ///\note After you restore a state, you cannot restore

     ///\note After you restore a state, you cannot restore

     ///a later state, in other word you cannot add again the edges deleted

     ///a later state, in other word you cannot add again the edges deleted

     ///by restore() using another SnapShot instance.

     ///by restore() using another SnapShot instance.

     ///

     ///

     class SnapShot 

     class SnapShot 

     {

     {

       SmartGraph *g;

       SmartGraph *g;

     protected:

     protected:

       friend class SmartGraph;

       friend class SmartGraph;