Here is what the experimental grammar had (the official grammar is very similar):
abs_term_1 = expr:(abs_tag_term / sumti / termset) {return _node("abs_term_1", expr);}
abs_tag_term = expr:((!gek tag free* !selbri !gek_sentence (sumti / joik_ek sumti_3 / KU_elidible free*) / !gek (sumti free*) CO_clause tag free* / NA_clause free* KU_clause free* / !selbri !gek_sentence !ek !joik_jek !gihek NA_clause free* KU_elidible free* / SOI_clause free* subsentence SEhU_elidible free*) (joik_jek abs_tag_term)*) {return _node("abs_tag_term", expr);}
Clearly, you can see that "sumti" occurs two times and what is unfortunate is that it appears on another level of the tree if a bare non-tagged sumti is used (the first, "abs_term_1 = " line above). More precisely, when FA is not used sumti is the direct child of abs_term_1 node whereas with FA it becomes a direct child of abs_tag_term which is a direct child of abs_term_1.
So {i fa mi prami} and {i mi prami} produce syntactic trees with a different hierarchy!
I think that no matter whether sumti is tagged with FA or not it should appear on the same level.
Existing tricks with PEG make the notion of sumti obscure.
Every sumti should take a certain position.
Not only we need a unambiguous grammar, we need a cleancut hierarchy.
So this is what I propose:
abs_term_1 = expr:(abs_tag_term / termset) {return _node("abs_term_1", expr);}
abs_tag_term = expr:((!gek tag free* !selbri !gek_sentence (sumti / joik_ek sumti_3 / KU_elidible free*) / !gek (tag / FA_elidible) free* !selbri !gek_sentence (sumti) / !gek (sumti free*) CO_clause tag free* / NA_clause free* KU_clause free* / !selbri !gek_sentence !ek !joik_jek !gihek NA_clause free* KU_elidible free* / SOI_clause free* subsentence
FA_elidible = expr:(FA_clause?) {return (expr == "") ? ["FA"] : _node("FA", expr);}
As you can see, "sumti" disappeared. I added "FA_elidible" which can fire only if a sumti is present (but not KU_elidible otherwise the grammar will crash).
Probably, " joik_ek sumti_3 " should also be copied to this FA_elidible part, right? I'm not an expert in all these things.