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Güldemann (2018), p. 188. Güldemann, Tom (2018). "Historical linguistics and genealogical language classification in Africa". In Güldemann, Tom (ed.). The Languages and Linguistics of Africa. The World of Linguistics. Vol. 11. Berlin: De Gruyter Mouton. pp. 58–444. doi:10.1515/9783110421668-002. ISBN978-3-11-042606-9. S2CID133888593.