non-numeric bar "numbers"

[Paul Scott]

Some show music uses more than just numbers for bar identification for 
certain bars.  For example an introduction might be "numbered" A, B, C, 
etc. and then inserted bars may be "numbered" 67A, 67B, etc.  What would 
it cost to sponsor something like:  currentBarNumber =  (#number . 
#letter) where if the letter was present it would be incremented instead 
of the number?

Thanks,

Paul Scott