exercise notation

[lilypond]

Hello,

Singing in a coral I would like to exercise the different parts  
(soprano, alto, bass).
So I would like to generate from a single .ly-file, multiple midi-files:
- all parts together (which is allready working)
- only soprano part
- only alto part
- only bass part
With a small kind of switch.

My preference would be using boulean variables for example:
* midi-soprano = true/false
* midi-alto = true/false
* midi-bass = true/false

How can I implement something like this, in the attached example?

Thanks a lot in advance!!

Regards, Jose