optional transposition triggered by an external file?

[Marc Hohl]

Hello list,

for my current project I have some scores located in
individual files:

A.ly
B.ly
C.ly

which can be compiled independently, and a latex file
containing calls like this

\lilypondfile{A.ly}

[... some text ...]

\lilypondfile{B.ly}

[... even more text ...]

\lilypondfile{C.ly}

This works fine.

Now I want to include an appendix for transposing instruments
(Saxophone in e flat etc.) where all the files are transposed
accordingly.

Now my (probably way too complicated and crude) idea sounds like this:

I need a way to define an optional variable that controls (if defined)
the transposition so I can say, for example:

\begin{appendix}
\chapter{For Bb instruments}
\begin{lilypond}
#(define transposeTo d)
\include{A.ly}
\end{lilypond}

\begin{lilypond}
#(define transposeTo d)
\include{B.ly}
\end{lilypond}

...
\end{appendix}

As an alternative, the #(define ...) stuff could be hidden in a file
transposeToBes.ily and the above calls change to

\begin{appendix}
\chapter{For Bb instruments}
\begin{lilypond}
\include{transposeToBes.ily}
\include{A.ly}
\end{lilypond}

\begin{lilypond}
\include{transposeToBes.ily}
\include{B.ly}
\end{lilypond}

...
\end{appendix}

Is this feasible, or am I overlooking the obvious?
I like the idea of having compilable standaloe files for each
piece of music, so this is what I want to achieve.

Regards,

Marc