Changing the bar-number string

[Francesco Guglie]

Hi everybody!
Since I'm having some problems posting my mail to the mailing-list through my 
mail-client, I'm trying to resend this one through the gmane interface. 

I'm typesetting an orchestral score with repeated sections inside each 
movement; I'd like to change the bar-numbers of a repeated section so that it 
shows the bar-number of the first time with a "bis" added. 
I'll try to explain it better with an example: the first number of ther 
repeated section is "34"; when it is repeated, I'd like to have "34bis" and 
so on for all the bar numbers of the section.
I've tried to use the properties of BarNumber context, but with no success. 
I'm using lilypond 2.11.41.
Thank you all!

Francesco Guglie