Fine et al. 2022, Corollary 1.8, p. 11. Fine, Benjamin; Moldenhauer, Anja; Rosenberger, Gerhard; Schürenberg, Annika; Wienke, Leonard (2022), Geometry and Discrete Mathematics: A Selection of Highlights, De Gruyter Textbooks (2nd ed.), Walter de Gruyter, ISBN9783110740783