Vertical order of grobs

[Reinhold Kainhofer]

I'm currently trying to fix the vertical order of figured bass. However, I 
don't see where in the engraver the vertical order of the items is actually 
determined...

The problem is for the following snippet:

\version "2.13.11"
<<
  \context Voice { \clef bass
    c8 c4 c8
  }
  \figuremode { \set Staff.useBassFigureExtenders = ##t
    <6 4>8 <4 3>4
  }
>>

The output should be:

6
4----
    3

while it currently is:

6   3
4----

(There are also some other issues resulting from the internal structure, so 
I'll do a complete rewrite...). 

Where exactly can I set in figured-bass-engraver.cc (or what object is 
responsible for it?) that the 3 should be placed below the extneder of the 
4??? 
All I need are some pointers to understand the vertical stacking order. 

Thanks,
Reinhold

-- 
------------------------------------------------------------------
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