From the introduction: "[...] we discuss the morphology and segmental morphophonemics of Mixtec in a degree of detail which has never been done before. The most novel aspect of our work below, for Mixtec studies, is that we posit underlying forms for the two tense prefixes and then account for the variety of surface forms. Since in many--indeed, in a majority--of situations the underlying segments are not present in the surface form, the analysis is not immediately obvious."
Gittlen, Laura and Marlett, Stephen A.
"Ñumí Mixtec syllable structure and morphology,"
Work Papers of the Summer Institute of Linguistics, University of North Dakota Session: Vol. 29, Article 3.
Available at: https://commons.und.edu/sil-work-papers/vol29/iss1/3