aspect Undo {
	public static boolean Program.DEBUG_UNDO = false;
	private static boolean Program.RECORDING_CHANGES = false;
	public static long Program.undoActionCount = 0;
	
	public static boolean Program.isRecordingASTChanges() {
		return RECORDING_CHANGES;
	}
	
	
	public static void Program.startRecordingASTChangesAndFlush() {
		RECORDING_CHANGES = true;
		undoStack = new Stack<ASTModification>();
	}
	public static void Program.stopRecordingASTChangesAndFlush() {
		RECORDING_CHANGES = false;
		undoStack = new Stack<ASTModification>();
	}
	
	// CheckPoints
	
	private static ASTUndoCheckPoint Program.lastCheckPoint = null;
	public static ASTUndoCheckPoint Program.createASTUndoCheckPoint() {
		if (!isRecordingASTChanges())
			startRecordingASTChangesAndFlush();
		ASTUndoCheckPoint cp = new ASTUndoCheckPoint();
		lastCheckPoint = cp;
		undoStack.push(cp);
		return cp;
	}
	
	class ASTUndoCheckPoint extends ASTModification {
		@Override
		public void undo() {
		}
		@Override
		public void redo() {
		}
		@Override
		public String toString() {
			return "ASTUndoCheckPoint";
		}
	}
	
	// General ASTModification
	
	public abstract class ASTModification {
		public abstract void undo();
		public abstract void redo();
	}
	
	/*
	 * Undo Stack
	 * 
	 * problem with non-static stack: creating new subtrees that are not attached to the
	 * main tree with a Program root node -> no undo in the new subtree
	 */
	private static Stack<ASTModification> Program.undoStack = new Stack<ASTModification>();
	
	public static Stack<ASTModification> Program.getUndoStack() {
		return undoStack;
	}
	
	public static Stack<ASTModification> Program.cloneUndoStack() {
		return (Stack<ASTModification>) (undoStack != null ? undoStack.clone() : null);
	}
	
	// Undo-ing
	
	public static void Program.addUndoAction(ASTModification action) {
		undoStack.push(action);
		undoActionCount++;
	}
	
	public static void Program.undoAll() {
		undoUntil(null);
	}
	
	public static void Program.undoUntilLastCheckPoint() {
		undoUntil(lastCheckPoint);
	}
	
	public static void Program.undoUntil(ASTUndoCheckPoint cp) {
		if (DEBUG_UNDO) System.out.print("Undoing " + undoStack.size() + " actions... ");
		while (!undoStack.isEmpty()) {
			ASTModification mod = undoStack.pop();
			mod.undo();
			if (cp != null && cp.equals(mod))
				break;
		}
		if (DEBUG_UNDO) System.out.println("done.");
		if (DEBUG_UNDO) System.out.println("Total undo actions: " + undoActionCount);
	}
	
	
}