https://www.researchgate.net/publication/261320155_Typology_and_reconstruction_The_consonants_and_vowels_of_Proto-Indo-European

implosive stop to voiced stops
plain to breathy based on african languages

e o reinterpreted as a ā with accent, front back non phonemic distinction comparison of similar systems

addressing of niche problems

some comparison of protoindoeuropean reconstructions