Parser variables, properties and contexts, rooky developer question.

[Ian Hulin]

Hi all,
I've been grubbing around, grepping my git repository and I need a bit 
of steering from some of you who've been around the lilypond source 
longer than me.

I'm trying to design a solution to the original request for tracker 714, 
which asked to be able to specify an alternate output filename from the 
code.  I don't think this is completely feasible but feel we should be 
able to support changing the output suffix for midi files, which are 
produced on a per \score block basis, so I would need to be able to 
define a property to do this for \score which would pick the value of 
the output-suffix parser variable as a default and would allow it to be 
user-settable for the scope of the \score.  For example:

% In file VW.ly
#(define output-suffix "Five-songs")
\book {
...
\bookpart
{
...
  \score \with {midi-output-suffix="Dark-Eyed-Sailor"}
  {
    \midi{}
    \layout{}
    music...
  }
%above produces midi file VW-Dark-Eyed-Sailor.mid(i)
% pdf output is going to VW-Two-Songs.pdf
\bookpart {
...
  \score \with {midi-output-suffix="Lovely-On-The-Water"}
  {
    \midi{}
    \layout{}
    music...
  }
%above produces midi file VW-Lovely-On-The-Water.mid(i)
% pdf output is still going to VW-Two-Songs.pdf
}

The parser variable is currently scoped for each source file unit or 
\book block.
The \bookpart block would need to pick up the value of output-suffix as 
defined for the current \book block and store it as an internal 
property.  Each \score block could then pick up the \bookpart 
outpt-suffix setting and copy it to its own midi-output-suffix property 
which would be settable via the user interface.

So finally, here's my questions:
 whereabouts in the code would I declare the new midi-output-suffix 
property?
 would the code to get \bookpart to inherit the output-suffix parser 
variable value and \score to inherit the \bookpart value be C++ or 
Scheme, and in which bits of code?

Thanks for any ideas/help in advance,
Cheers,
Ian Hulin