1. Enter all the code as per the original into a "notes" file keeping
the five sections described above in separate definitions (macros):
% file: dvorak_trumpet_original_part.ly
% apply appropriate \relative commands below
emusici = { <first half in E> }
esmusic = { <2 measures in Es> }
emusicii = { <16 measures in E> }
cmusic = { <16 measures in C> }
emusiciii = { <rest in E> }
2. Assemble the piece in the same file with:
music = { \emusici
\transpose e es \esmusic
\emusicii
\transpose e c \cmusic
\emusiciii
}
I consider this still too complicated. The right solution IMHO is to
use \tag commands (e.g. \tag 'score, \tag 'trumpetI, \tag 'trumpetII,
etc.) which are then filtered out accordingly.