// _reversibility_stack = new FastStack();
_reversibility_stack = new UndoStack();
_goal_stack = new GoalStack(_reversibility_stack);
_propagation_queue = new FastQueue();
// _goal_CP_marker = new GoalDisplay(this, "ChoicePointMarker");
// _undo_CP_marker = new UndoImpl();
// _save_goal = new GoalDisplay(this,"Save");