Lightweight XML writer not needing external libraries?

I have been looking for means to write XML files when I come across this thread which features a lightweight XML parser written in Javascript for Unity3D. I have the impression that including the MONO library for writing/reading XML would add an additional 1 MB of download (and the application is designed for web-player/iPhone deployment).

Is there any lightweight solution which does not require external libraries?

Check out this topic on the Unity forums.