This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
You'd be more likely to get help if you actually posted the code of the callback function. In the code you've provided node is a variable being passed as a function parameter, though I have no idea what it means or what it does, the same as if you picked any other bit of code at random off the internet and posted it with no context. What are you trying to do? What are you using to do it?