// _execution_stack = new FastStack();
// _alternative_stack = new FastStack();
// _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();