Clefs for cue notes: How to extract a grob property in an engraver?

[Reinhold Kainhofer]

Hi all,
I'm currently working on automatic cue clefs (i.e. \cueDuring in different 
clefs than the containing voice, e.g. using a violin or flute cue inside the 
cello part).
My current state is up at rietveld at:
http://codereview.appspot.com/2588042

As the cue clefs should not override the staff's clef (in particular, at the 
beginning of a piece and after a line break the staff's normal clef should be 
shown and the smaller cue clef should be shown just right before the cue 
notes), I'm adding a new grob CueClef. After the cue notes, a second clef to 
reset to the instrument's clef also needs to be added.
I have added a Cue_clef_engraver to take care of this.

An example output of the current state is at:
   http://www.fam.tuwien.ac.at/~reinhold/LilyPond/cue-clef.pdf

I'm currently struggling with two/three issues:
1) At the end of the cue notes I need to reset the clef to the clef of 
   the containing voice. Unfortunately, the clef glyph of the containing 
   voice is not stored in a context property, but as the glyph property 
   of the Clef grob. How can I retrieve that grob property from within an 
   engraver?
2) I tried to add appropriate space-alist entries to scm/define-grobs.scm and
   also add the cue-clef and cue-end-clef to the break-align-orders.
   Unfortunately, the cue clefs are not positioned correctly:
    -) a cue clef at the start of a measure touches the barline (no space!)
    -) an ending cue clef is printed AFTER the barline
    -) an ending cue clef at a line break is repeated in the new line
3) I'm getting a programming error:
     programming error: No spacing entry from Item to `cue-clef'
   I tried to find out which spacing entry is missing, but AFAICS, I have
   added all neccessary spacings to the appropriate spacing-alist entries...

I hope someone can help me with these issues.
  
Thanks,
Reinhold
-- 
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Reinhold Kainhofer, address@hidden, http://reinhold.kainhofer.com/
 * Financial & Actuarial Math., Vienna Univ. of Technology, Austria
 * http://www.fam.tuwien.ac.at/, DVR: 0005886
 * LilyPond, Music typesetting, http://www.lilypond.org