Transposition with Enharmonic Changes and ``Late-Binding'', now with error reporting!

gistfile1.ly

\version "2.17.22"

beginning = \relative c'' {
  \key as \major
  c1
}

middlesection = \relative gis' {
  \key e \major
  gis1
}

end = \relative c'' {
  \key as \major
  c1
  \bar "|."
}

% ``Late-bind'' variables using an arity-0 music function that looks up values
% at function call time instead of variable assignment time.
late =
#(define-scheme-function
  (parser location embedded-lilypond)
  (list?)
  (define-music-function
    (parser location)
    ()
    (eval embedded-lilypond
	  (interaction-environment))))


music = \late #'#{
  \beginning
  \middlesection
  \end
  #}

\score {
  % The music without the enharmonic change---complex key signature!
  \new Staff \transpose as g \music
  \layout {}
}

% Apply enharmonic transposition to \middlesection.
middlesection = \transpose e fes \middlesection

\score {
  % Because variables in \music are ``late-bound'', the enharmonic
  % transposition is now respected.
  \new Staff \transpose as g \music
  \layout {}
}