diatonicTranspose question

[address@hidden]

I've been trying out John Mandereau's diatonicTranspose feature.  It's *very* 
useful, but I've hit a snag.  When I try to transpose to -7 the octaves start 
to go wrong.  Known limitation?  Bug?  Pilot error?  See example:

-David

%%% BEGIN LILYPOND %%%

%%% BEGIN JOHN MANDERAU'S CODE %%%

#(define (true-quotient n d)
  "Return the one true quotient in integer division of n by d, i.e.
return the integer q so that there is a unique integer r that
satisfies  n = qd + r  and  0 <= r < |d|"
  (if (>= n 0)
   (quotient n d)
   (- (quotient n d) 1)))

#(define (ly-pitch->modal-pitch ly-pitch scale tonic-c-diff)
  "Convert ly-pitch to a list (octave degree depresentation) which represents
the modal pitch in scale, with tonic-c-diff as the pitch diff from tonic to
middle C.  scale should be a notename->alteration alist of length 7 which
represents the alteration of each note with C as first pitch of the scale."
  (let* ((normalized-pitch (ly:pitch-transpose ly-pitch tonic-c-diff))
         (notename (ly:pitch-notename normalized-pitch)))
   (list
    (ly:pitch-octave normalized-pitch) ;; octave
    notename ;; degree
    ;; alteration
    (- (ly:pitch-alteration normalized-pitch) (ly:assoc-get notename scale)))))

#(define (degree-transpose modal-pitch degree-delta scale-length)
"Transpose modal-pitch (octave degree alteration) by degree-delta assuming
scale-length."
  (let* ((octave (car modal-pitch))
         (degree (cadr modal-pitch))
         (alteration (caddr modal-pitch))
         (relative-new-degree (+ degree degree-delta)))
   (list
    (+ octave (true-quotient relative-new-degree scale-length))
    (modulo relative-new-degree scale-length)
    alteration)))

#(define (modal-pitch->ly-pitch modal-pitch scale c-tonic-diff)
  "Convert modal-pitch -- a list (octave degree alteration) -- to a
standard pitch using scale and pitch diff from middle C to tonic.
scale should be a notename->alteration alist of length 7 which represents
the alteration of each note with C as first pitch of the scale."
  (let* ((octave (car modal-pitch))
         (degree (min 6 (cadr modal-pitch)))
         (alteration (caddr modal-pitch))
         (abs-alteration (+ alteration (ly:assoc-get degree scale))))
   (ly:pitch-transpose
    (ly:make-pitch octave degree abs-alteration)
    c-tonic-diff)))

#(define (lookup-music-property event type)
  "Return the first music property of the given type found in event,
or #f is no such property is found.  event should be music or a list
of music."
  (if (null? event)
   #f
   (if (list? event)
    (or
     (lookup-music-property (car event) type)
     (lookup-music-property (cdr event) type))
    (let ((p (ly:music-property event 'pitch)))
     (if (not (null? p))
      p
      (let* ((e (ly:music-property event 'element))
             (es (ly:music-property event 'elements))
             (p2 (if (null? e)
                  '()
                  (lookup-music-property e type))))
       (if (not (null? p2))
        p2
        (lookup-music-property es type))))))))

#(define (mode-transpose
          pitch-note1 scale1 degree-delta pitch-note2 scale2 music)
  (let ((tonic-diff1 (ly:pitch-diff
                      (ly:make-pitch 0 0 0)
                      (lookup-music-property pitch-note1 'pitch)))
        (tonic-diff2 (ly:pitch-diff
                      (lookup-music-property pitch-note2 'pitch)
                      (ly:make-pitch 0 0 0))))
   (music-map
    (lambda (event)
     (let ((p (ly:music-property event 'pitch)))
      (if (ly:pitch? p)
       (ly:music-set-property!
        event
        'pitch
        (modal-pitch->ly-pitch
         (degree-transpose
          (ly-pitch->modal-pitch p scale1 tonic-diff1)
          degree-delta
          (length scale1))
         scale2
         tonic-diff2)))
      event))
    music)))

modeTranspose =
#(define-music-function
  (parser location pitch-note1 scale1 degree-delta pitch-note2 scale2 music)
  (ly:music? list? number? ly:music? list? ly:music?)
  "Transpose music from scale1 on tonic pitch-note1
by degree-delta to scale2 on tonic pitch-note2."
  (mode-transpose
   pitch-note1 scale1 degree-delta pitch-note2 scale2 music))

diatonicTranspose =
#(define-music-function
  (parser location pitch-note scale degree-delta music)
  (ly:music? list? number? ly:music?)
  "Transpose music by degree-delta in scale on tonic pitch-note."
  (mode-transpose
   pitch-note scale degree-delta pitch-note scale music))

%%% END JOHN MANDERAU'S CODE %%%


\version "2.13.3"

pattern = \relative c''' { g c, b c }

\score {
  \relative {
    \new Staff {
      \pattern
      \diatonicTranspose c \major #-6 \pattern  %% This is OK
      \diatonicTranspose c \major #-7 \pattern  %% This goes wrong
    }
  }
}

%%% END LILYPOND %%%