> >If so, is the *-r/*-n part detachable from the stem in *-u, *-i and
> *-a?
>
> No. *-un- is a suffix. Suffixes are (C)VC(C).
>
Is it true that you have stems suffixed in *-un, *-in, *-an which
becomes your various n-stems, and that /u/, /i/, and /a/ become the
ablaut vowel, which means thematic stems are really *-u, *-i, *-a stems?
Because if so, there exists an n-suffix.