In reference to this discussion:
I solved almost all the problems, but one thing still remains obscure to me.
The difficulty is twofold, since it involves on the one hand creating the correct lookups for the font, on the other hand making sure that the text code applies them in turn correctly.
Now, (Lua)LaTeX with Fontspec and Babel allows you to choose among several options: 1) monotonic modern Greek, 2) polytonic modern Greek, 3) ancient greek (polytonic). In this case the code I use is the following:
I can't find specific tags related to ancient Greek.
I tried using the GRC and GRK specifications (in addition to PGR), which however do not seem to be correct otf tags.
In fact, a verification of the font tells me:
otfinfo -s SimonciniGaramondPro.otf
grek.GRC Greek / <unknown language> <=======
grek.GRK Greek / <unknown language> <=======
grek.PGR Greek / Polytonic Greek
latn.AZE Latin / Azeri
latn.CRT Latin / Crimean Tatar
latn.DEU Latin / German
latn.FRA Latin / French
latn.ITA Latin / Italian
latn.MOL Latin / Moldavian
latn.ROM Latin / Romanian
latn.TRK Latin / Turkish.
Now, is there any way out? Can't 'locl' lookups have Greek as their language and 'ancient' as specification?
What are the linguistic alternatives allowed?