More complex preprocessing with scheme

[Andrew Zah]

Hi all,

Is it possible to do something like have lilypond evaluate a Scheme file 
in order to do more complicated preprocessing of song files?

Ultimately I'd like to have a template and then include files like so, 
based on some logic:

% ...
\bookpart {
 \tocItem \markup "$SONG"
 \include "$FILE"
 % etc.
}
% ...

I know it's possible to achieve this with external tools, but I'd like 
to be able to simply run `lilypond` with some flags to change the output.

Thanks,
Andrew