Invalid overrides in ly/gregorian.ly?

[Patrick McCarty]

Hello,

After looking through source, I can't find a spot where
'extra-X-extent and 'extra-Y-extent are recognized, yet they are
documented in the grob interface and an 'extra-X-extent override is
used six times in ly/gregorian.ly.

If they are no longer functional, should a convert-ly rule be added for them?

Do 'minimum-X-extent and 'minimum-Y-extent replace these old
properties?  If not, which overrides should be used instead for
\virgula, \caesura, etc. in ly/gregorian.ly?

Thanks,
Patrick