For a teaching material I needed a good example of vocalic mutation of the root, aka Umlaut, and I got stuck at the fact that, while the Umlaut is often postulated for some reconstructed languages, there seem to be few, if any, good examples of this phenomenon in some contemporary living languages.
By Umlaut/vocalic mutation I mean a morpho-phonological mechanism such that the vowel of the root (or stem) is assimilated to the vowel of the ending. Note that I want the ending vowel to be preserved, therefore such examples as the Old English nom.sg. fōt ~ nom.pl. fēt are not accepted, because the ending that provoked the vowel mutation is never attested in the written sources.
Curisouly enough, the opposite phenomenon, i.e. the vocalic harmony (when it's the ending vowel to change accordingly to the vowel of the root), is well attested cross-linguistically, for example in Turkish. Not sure what general conclusion can be drawn here, but still...