\version "2.18.2"
% vim: filetype=lilypond:

% Substitute for \key\default that works with midi{}
reprintKey = \applyContext #(lambda (c)
     (define (same-scale-step? a b) (= (car a) (car b)))
     (let* ((pitch-alist (delete-duplicates!
                         (append (ly:context-property c 'keyAlterations)
                                  major)
                          same-scale-step?))
            (tonic (ly:context-property c 'tonic (ly:make-pitch 0 0 0)))
            (sev (ly:make-stream-event `(key-change-event)
                                       `((pitch-alist . ,pitch-alist)
                                         (tonic . ,tonic)))))
       (ly:broadcast (ly:context-event-source c) sev)))

% from "scm/define-context-properties.scm"
#(define (translator-property-description symbol type? description)
   (if (not (and
             (symbol? symbol)
             (procedure? type?)
             (string? description)))
       (throw 'init-format-error))
   (if (not (equal? #f (object-property symbol 'translation-doc)))
       (ly:error (_ "symbol ~S redefined" symbol)))
   (set-object-property! symbol 'translation-type? type?)
   (set-object-property! symbol 'translation-doc description)
   (set! all-translation-properties (cons symbol all-translation-properties))
   symbol)
#(translator-property-description 'printTransposition ly:pitch? "transposition for printing with autoTransposeEngraver")

% adapted from Jan-Peter Voigt's autoTransposeEngraver at openlilylib
printTranspositionEngraver = #(lambda (context)
  (let ((old-pitch-alist major)
        (old-transp (ly:make-pitch 1 0 0)))
     (make-engraver
      (listeners
       ((melodic-event engraver ev)
          (let ((transp (ly:context-property context 'printTransposition))
                (pitch (ly:event-property ev 'pitch)))
             (if (and (ly:pitch? pitch) (ly:pitch? transp))
               (ly:event-set-property! ev 'pitch 
                                       (ly:pitch-diff pitch transp)))))
       ((key-change-event engraver ev)
          (let ((transp (ly:context-property context 'printTransposition))
                (tonic (ly:event-property ev 'tonic
                         (ly:context-property context 'tonic ;for \key\default
                           (ly:make-pitch 1 0 0))))
                (pitch-alist (ly:event-property ev 'pitch-alist
                                old-pitch-alist)))
             (if (ly:pitch? transp)
               (begin
                 (ly:event-set-property!  ev 'tonic
                    (ly:pitch-diff tonic transp))
                 (set! pitch-alist
                       (ly:transpose-key-alist pitch-alist 
                          (ly:pitch-diff old-transp transp)))
                 (ly:event-set-property! ev 'pitch-alist pitch-alist)
                 (set! old-transp transp)))
             (set! old-pitch-alist pitch-alist)))))))
printTransposition = \with {
  \remove Key_engraver
  \consists \printTranspositionEngraver
  \consists Key_engraver
}
% ===================================================

\addInstrumentDefinition #"cl Bflat"
#`(
(printTransposition . ,(ly:make-pitch -1 6 FLAT))
(shortInstrumentName . "Cl. in Bb")
(clefGlyph . "clefs.G")
(middleCPosition . -6)
(clefPosition . -2)
(instrumentCueName . ,(make-bold-markup "Cl. in Bb"))
(midiInstrument . "clarinet")
)
\addInstrumentDefinition #"flute"
#`(
(printTransposition . ,(ly:make-pitch 0 0 0))
(shortInstrumentName . "Flute")
(clefGlyph . "clefs.G")
(middleCPosition . -6)
(clefPosition . -2)
(instrumentCueName . ,(make-bold-markup "Flute"))
(midiInstrument . "flute")
)


part = {
  g'4 b' d'' fis'' R1
  g'4 b' d'' fis'' R1
  g'4 b' d'' fis'' R1
  }
arrangement = {
   s1*2 \instrumentSwitch "cl Bflat" \bar"||"\reprintKey
   s1*2 \instrumentSwitch "flute" \bar"||"\reprintKey
   s1*2
   }
global = { \key g\major s1*4  }

\new Staff \with { \printTransposition }
  \new Voice << \global \arrangement \part >>