(De)activate lilypond-source snippets based on whether a variable is defined

[Hans Aikema]

As there are several advanced Lilypond programming experts on this group 
I'd like to know if any of you know a way to have  conditional 
activation of lilypond-sourcelines based on the existence of a defined 
variable.

The basic setup of my Lilypond scores is as follows:

Song.ly: A 'wrap it all up' lilypond source containing:
* Song meta-information (\header block)
* Definition of a variable to define the common shared items such as key 
and tempo
* A \include per voice to include the voice's music- and lyrics-variables
* A \include to include the score-layout (containing definitions to get 
a combined score, full-volume all parts midi and a per-voice highlighted 
midi (highlighted by reducing volume of the other voices)

Per voice a <voicename>.ly file containing:
* a variable definition to define the music: <voicename>Music
* a variable definition to define the lyrics: <voicename>Words

ScoreLayout.ly: I need one file for each possible score-setup, with a 
repeating and predictable structure only varying for the number of voices:
* \include of some global formatting options
* definition of variables for the midi-instruments and volume-levels of 
'full volume' and 'reduced volume' voices
* variable-definition of the combined music with all voices at 'full 
volume' (allMusic)
* per voice a variable-definition of the combined music with all voices 
at 'reduced volume' except for the voice to highlight
* a \score containing a \layout on which New_dynamic_engraver is removed 
(because dynamics are used as a hack for midi-volume
* a \score per voice using that voice's variable definition that joins 
the voice at full volume with the other voices at reduced volume 
containing a \midi {}

Now where I start to annoy myself is that whenever I encounter a new 
parts-setup I have to create a new layout (properly named like 
layout-SATB-SATB-lyrics.ly, layout-SoSATB-SoSATB-lyrics.ly, 
layout-SoSAATB-SoSAATB-lyrics.ly, layout-SoSATB-SoST-lyrics.ly etc. 
etc). What I'd like to do for ScoreLayout.ly is something like (the 
'optionally include blocks' marked with %if and %endif):

allMusic = {
    \new ChoirStaff <<
        %if defined variable soloOneMusic
            \new Staff = soloOne \with {
                  midiMinimumVolume = \volFullMin
                  midiMaximumVolume = \volFullMax
            }<<
                \new Voice = "voiceSoloOne" { << \global \soloOneMusic >> }
            >>
        %endif
        %if defined variable sopranoMusic
            \new Staff = soprano \with {
                  midiMinimumVolume = \volFullMin
                  midiMaximumVolume = \volFullMax
            }<<
                \new Voice = "voiceSoprano" { << \global \sopranoMusic >> }
            >>
        %endif
        %if defined variable sopranoTwoMusic
            \new Staff = sopranoTwo \with {
                  midiMinimumVolume = \volFullMin
                  midiMaximumVolume = \volFullMax
            }<<
                \new Voice = "voiceSopranoTwo" { << \global 
\sopranoTwoMusic >> }
            >>
        %endif
        %if defined variable soloOneWords
            \new Lyrics \with {alignBelowContext=solo} \lyricsto 
voiceSolo \soloWords
        %endif
        %if defined variable sopranoWords
            \new Lyrics \with {alignBelowContext=soprano} \lyricsto 
voiceSoprano \sopranoWords
        %endif
        %if defined variable sopranoTwoWords
            \new Lyrics \with {alignBelowContext=sopranoTwo} \lyricsto 
voiceSopranoTwo \sopranoTwoWords
        %endif
    >> % end of ChoirStaff
}
soloOneHighlight = {
    \new ChoirStaff <<
        %if defined variable soloOneMusic
            \new Staff = soloOne \with {
                  midiMinimumVolume = \volFullMin
                  midiMaximumVolume = \volFullMax
            }<<
                \new Voice = "voiceSoloOne" { << \global \soloOneMusic >> }
            >>
        %endif
        %if defined variable sopranoMusic
            \new Staff = soprano \with {
                  midiMinimumVolume = \volReduceMin
                  midiMaximumVolume = \volReduceMax
            }<<
                \new Voice = "voiceSoprano" { << \global \sopranoMusic >> }
            >>
        %endif
        %if defined variable sopranoTwoMusic
            \new Staff = sopranoTwo \with {
                  midiMinimumVolume = \volReduceMin
                  midiMaximumVolume = \volReduceMax
            }<<
                \new Voice = "voiceSopranoTwo" { << \global 
\sopranoTwoMusic >> }
            >>
        %endif
        %if defined variable soloOneWords
            \new Lyrics \with {alignBelowContext=solo} \lyricsto 
voiceSolo \soloWords
        %endif
        %if defined variable sopranoWords
            \new Lyrics \with {alignBelowContext=soprano} \lyricsto 
voiceSoprano \sopranoWords
        %endif
        %if defined variable sopranoTwoWords
            \new Lyrics \with {alignBelowContext=sopranoTwo} \lyricsto 
voiceSopranoTwo \sopranoTwoWords
        %endif
    >> % end of ChoirStaff
}
%if defined variable soloOneMusic
\score {
    \unfoldRepeats << \global \soloOneHighlight >>
    \midi {}
}
%endif

etc.etc.

Is this somehow easy to accomplish? Or is it better to just keep up 
adding layouts using manual labor each and every time I encounter a 
different setup (more than the regular SATB-voices and/or one or more 
solos)?

regards,
Hans Aikema