\version "2.16.0"

% tonic is equal - octave is not checked!
#(define (ly:tonic=? p1 p2)
   (or
    (and (ly:pitch? p1) (ly:pitch? p2)
         (= (ly:pitch-notename p1) (ly:pitch-notename p2))
         (= (ly:pitch-alteration p1) (ly:pitch-alteration p2))
         )
    (and (eq? #f p1) (eq? #f p2))
    ))

%%%%%%%%%%% don't watch this hack ... just to display "real" notenames here ...
% pitch-notename
#(define (nona pp)
   (if (ly:pitch? pp)
       (let ((str "?"))
         (define (search pl pp)
           (if (pair? pl)
               (let* ((pr (car pl))
                      (pq (cdr pr)))
                 (if (ly:tonic=? pq pp)
                     (set! str (car pr))
                     (search (cdr pl) pp))
                 )
               ))
         (search pitchnames pp)
         str
         )
       "?")
   )
%%%%%%%%%%%%%%

% engraver to ...
% recognize every change of instrument transposition
#(define-public keyT
   (lambda (context)
     (let ((tonic (ly:make-pitch 0 0 0))
           (sig '())
           (last-pitch (ly:make-pitch 0 0 0))) ; no transp assumed

       `((process-music
          . ,(lambda (trans)
               (let ((current-pitch (ly:context-property context 'instrumentTransposition)))
                 ; if pitch (instrument transposition) has changed, create new time signature
                 (if (and (not (ly:tonic=? last-pitch current-pitch))
                          (ly:pitch? current-pitch))
                     (begin
                      ; how to create a time signature grob?
                      ; how to create the needed properties 
                      ; -> concert pitch = bes maj, instr transp bes
                      ; => should give c maj key sig with key cancelation
                      (let ((time-sig (ly:engraver-make-grob trans 'KeySignature '())))
                        (ly:message "What to do here?")
                        )
                      (let ((time-sig (ly:engraver-make-grob trans 'TextScript '())))
                        (set! tonic (ly:context-property context 'tonic))
                        (set! sig (ly:context-property context 'keySignature))
                        (ly:grob-set-property! time-sig 'text (format "~A>~A" (nona last-pitch) (nona current-pitch)))
                        (set! last-pitch current-pitch)
                        )
                      ) ; if #t
                     ; if #f not set
                     ) ; fi
                 )))

         (stop-translation-timestep
          . ,(lambda (trans)
               (set! tonic (ly:make-pitch 0 0 0)))))
       )
     ))


% use this engraver
\layout {
  \context {
    \Staff
    \consists #keyT
  }
}

% a test score
\score {
  \relative c'' {
    \key bes \major
    bes2 a \transposition bes c^"we should get a c maj with key cancel." b
  }
  \layout { }
  \midi { }
}