I don't know, if this matters, but in ancient notation, there
definitely is an "ij" ligature (as well as an "IJ" ligature) that is
quite often used as a textual repetition directive in lyrics. I
hope the according definitions at the very beginning of
ly/gregorian-init.ly will still work and handled as ligatures?
Ah, we use different terminology. With *ligature* I mean the process
of mapping the input character sequence `I' + `J' to `IJ', not the
*ligature glyph* `IJ' -- this is still there, of course.