 Klein's group-theoretical view of geometry enjoyed much favor among mathematicians and philosophers. It achieved a major success when Minkowski (1909) showed that the gist of Einstein's special theory of relativity was the (spacetime) geometry of the Lorentz group, an essential result that Klein (1911) lived to enjoy. It implies that the recent debate on the priority of Minkowski chronogeometry over Lorentz invariance or vice-versa is utterly idle, for these are logically equivalent and thus, in effect, two sides of the same coin (as explained by Acuña (2016)). However, Klein's Erlangen program failed to cover the differential geometry of Riemann (§5), which Einstein (1915, 1916) placed at the core of his general theory of relativity.