guide finger

[pabuhr]

In *classical* guitar-music, there is a finger augmentation called a "guide
finger" indicating left-hand shifting. Nick Payne uses a mechanism based on the
glissando.

I found this code written for a finger shift on a violin, which I like because
it handles finger location and color, etc. However, for guitar, there are 3
kinds of finger shift: up, neutral and down, corresponding to about 30 degrees
up, flat and 30 degrees down for the "-" character.

So I can't make the changes to do what I want. 8-( I have sketched out what I
would like, which is 2 default parameters, the guide character and the rotation
amount, and to rotate this character before concatenating it with the string
number.

Any help would be greatly appreciated.

============================================================================

\version "2.14.2"

gn = #(define-music-function (parser location fingering) (ly:music?)
;;   #:properties ((guide-char #x2013) (rotate-guide 0))
   (let ((music (make-music 'FingeringEvent))
         (finger (ly:music-property fingering 'digit)))
     (set! (ly:music-property music 'tweaks)
           (acons 'stencil
                  (lambda (grob)
                    (grob-interpret-markup grob
                                           (make-concat-markup
                                            (list
;;                                             (rotate rotate-guide 
(make-char-markup guide-char) )
                                             (make-char-markup #x2013)
                                             (number->string finger)))))
            (ly:music-property music 'tweaks)))
    music)
)
% gu = \override #'(rotate-guide 30) \gn
% gd = \override #'(rotate-guide -30) \gn

\relative c' {
  \override Fingering #'color = #red
  \set fingeringOrientations = #'(left)
  <c-\gn-3>4
  c4^\gn^3
  c4-\gn-3
  c4_\gn_3
  \set fingeringOrientations = #'(right)
  <c-\gn-3>4

%  <c-\gu-3>4
%  c4^\gu^3
%  c4-\gu-3
%  c4_\gu_3
}