figured bass : flat over figure

[Jean-marc LEGRAND]

hi list !

using XP lily 2.8.3

I've discovered a odd thing in figured bass, but it surely has a scientific 
explanation !


In this example :

\version "2.8.3"
theorbefigures = \new FiguredBass
\figuremode {
\override FiguredBass.BassFigure #'font-size = # -2
<_->2 <7 _->
}
theorbe = \new Voice {
\clef bass
\relative c {
d4 g, a2
}}
\score {
\new StaffGroup <<
\theorbefigures
\new Staff <<\set Staff.midiInstrument = "cello" \theorbe >>
        >>
\layout { }
\midi { \tempo 4=80}
     }


the <7 _-> gives me a 7 UNDER the flat sign.

When I change the figured bass to :
<6>2 <7 _->

the  <7 _-> gives me a 7 ABOVE the flat sign, and that's what I want.

Does anyone know how to get the flat under the figure, even if there's another 
flat before ?

Regards.

JMarc