“…The links between the theory of racks and quandles on the one hand, and the theory of groups on the other, are tight. In particular, the development of a higher covering theory of racks and quandles started in [28,29] is to be viewed in parallel with corresponding breakthroughs in group theory. Based on the development and applications of the categorical Galois theory of group, [17,19,18,21], the classical and higher Hopf formulae [10,5] were rediscovered (see [20]) and generalized as in [13].…”