Completely separating notation from output logic

[Michael Ellis]

I'm trying to develop an include file with Scheme functions that will
allow notation files to contain nothing more than a header block and
music for one or more voices.   Below is what I've got so far.  What
I'm finding is that I can easily define a music function that appends
voices to a list and a corresponding function that retrieves them in
order within the context of  a \score block.   What doesn't work so
far  is trying to generate the score block itself.  I get an error
complaining about "Unexpected \score".  I'm assuming that's because a
\score isn't a music type.  Is that right?  Also, what I'm doing now
feels a bit hackish.  Is there a more elegant approach?

Thanks,
Mike

%% To go into an include file, e.g. "myinclude.ly" ...
\version "2.12.3"
%% Append music for one voice to a list.
appendVoice = #(define-music-function (P L ml m) (list? ly:music?)
    (append! ml m))

%% Insert a voice in a score block
newvoice = #(define-music-function (P L M) (ly:music?)
    #{
        \new Voice $M
    #})

%% Insert list of voices to a score block
allvoices = #(define-music-function (P L  mlist) (list?)
    (if (eq? mlist '())
    #{  #}
    (let ((v (car mlist)))
        #{
            \newvoice  $v
        #}
        (allvoices (cdr mlist)))))

%% Generate a score block containing voices from a list.
thescore = #(define-music-function (P L mlist) (list?)
    #{
        \score {
            <<
            \allvoices \myvoices
            >>
        }
    #})


%% ---------------------------------------
%% To go into a notation file

%\include "myinclude.ly"

#(define myvoices '())

\appendVoice \myvoices \relative c' { e f g a }
\appendVoice \myvoices \relative c' { c d e f }

%% THIS WORKS
\score {
    <<
    \allvoices \myvoices
    >>
}

%% THIS DOES NOT
\thescore \myvoices


Cheers,
Mike