Re: I don't understand
> Why did someone feel the need to rewrite
> the C pre-processor?
CPP is not a good general-purpose macro processor.
The question is why would one reimplement m4?
GNU m4 (http://www.gnu.org/software/m4/).
Re: GNU screen
...or look at the Freshmeat project page (http://freshmeat.net/projects/screen/).