In Maga Rukai, an Austronesian language spoken in Taiwan, certain prefixes show an alternation between a mid vowel and a high vowel. What determines whether the prefix vowel is mid or high? (Only one rule is needed to cover all the examples.) What kind of rule is this? (Source: Hsin, Tien-Hsin, Aspects of Maga Rukai phonology. Doctoral dissertation, The University of Connecticut, 2000.)