Warnung (aus John Conway, Math. Intelligencer 23 (2001), no. 2, pp. 8–9): Coxeter came to Cambridge and gave a lecture in which he stated a problem for which he gave proofs for selected examples, and he asked for a unified proof. I left the lecture room thinking. As I was walking through Cambridge, suddenly the idea hit me, but it hit me while I was in the middle of the road. When the idea hit me I stopped and a large truck ran into me. . . . So I pretended that Coxeter had calculated the difficulty of this problem so precisely that he knew that I would get the solution just in the middle of the road. . . . Ever since, I’ve called that theorem “the murder weapon”. One consequence of it is that in a group if a^2 = b^3 = c^5 = (abc)^{-1}, then c^610=1.