How to do simple arithmetic on bar numbers?

[Graham King]

I'm preparing a modern-notation edition of a piece of choral renaissance
polyphony, in which the maxima at the end of each section is notated as
a neomensural "\maxima*n/m" (where, for each part, n,m are chosen to
cause all the parts to end the section together).  

Example lilypond code and the result are attached.

Bar-numbering is attached to the top part in the score so, when that
part's maxima starts early and other parts are still moving, the other
parts sometimes have "extra" bars of music.  As a result, I have to
reset the bar numbering manually afterwards (see line 27 of the example
attached):

       \set Staff.currentBarNumber = #4

Rather than specifying a static bar number, how could the following
pseudocode be programmed?

       \set Staff.currentBarNumber = Staff.currentBarNumber + x

where, in this example, x = 1

As you can tell, I'm not a Scheme programmer!  Thanks for any help you
can offer.
-- 
Graham King <address@hidden>

test_final_maxima.ly
Description: Text document

test_final_maxima.png
Description: PNG image