Underlined + double underlined fingering instructions for accordion scores

[Ledocq-Boccart]

Hello!

For left hand keyboard of accordion, fingering instructions are annoted 
with figures as 2,3,4,5 for the middle button row; with simple 
underlined figures (_2_, _3_, _4_, _5_),  the following row , and 
double-underlined figures for the last bass button row (I caanot give 
example with the font of the editor of my e-mail text processor).

I tried with
c^ \markup { \underline 2 }
in a score, for example, but without succes (I did not find any 
\underline instruction.in the doc)

Does this topics have already been solved by Lilyponders?

Thank you for answering.

Charlie