public SimpleHistoryNode(ICommand lastExisingCommand, SimpleHistoryNode lastExisingState)
 {
     PreviousCommand = lastExisingCommand;
     PreviousNode = lastExisingState;
 }
 void Init()
 {
     CurrentState = new SimpleHistoryNode();
     HeadNode = CurrentState;
     LastNode = CurrentState;
     Length = 0;
 }