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When Xfce 4.2 first came out I decided I wanted to have the compositor (it wasn't built by default). Knowing nothing about ABS but it's existance I read a few pages of the docs; and within 40 minutes I had rebuilt xfwm4 with compositor and was using it. And if I want to switch to their build, I just uninstall it and sync it with the repo server in pacman!
Unbelievably handy for the occasional tweak!