The Bantu languages are listed in such a way that almost every language is placed under a unique subheading. For example, Zulu and the other Nguni languages are listed separately from each other, as are the Shona and Sotho-Tswana languages. Most language subheadings include only a single language, though the subheading itself (e.g. Nguni) may be duplicated several times for related languages. Is this classification scheme a reflection of any published work? Thanks!