Ambiguous f. (forma or filius) needs a warning
Sometimes it is impossible to say just by looking at a name-string, if f.
in it belongs to an author as filius
(son of) or it belongs to the following epithet and designates its rank as forma
:
Aus bus L. f. cus
We decided that we create less damage, if we associate f.
with author as filius, than potentially creating a wrong rank for the infraspecific epithet. However we do not explicitly say that there is an ambiguous situation. We need to create a new level 2 warning "Ambiguous f. (forma or filius)" for such occasions.