Repeat count as "ij" in Gregorian chant

[Dan Eble]

Please see merge request 1235 [1] for an image of a repeat count for Gregorian 
chant that is generated according to \repeat in the ly code.

I learned of this notation only recently and I don't expect to use it myself 
except as a way to test LilyPond's data flow.  Is anyone here interested in 
specifying the minimum features that this counter must have to deserve a place 
in LilyPond?  I already know about the conflict between using BarLine or 
BreathingSign for divisions, and I hope to resolve that eventually.  What about 
the counter per se?

Thanks and regards,
— 
Dan

[1] https://gitlab.com/lilypond/lilypond/-/merge_requests/1235