 The thesis of Alexander Grothendieck's student Hoàng Xuân Sính was an in-depth study of 2-groups under the moniker 'gr-category'.The general definition of "n"-group is a matter of ongoing research.However, it is expected that every topological space will have a "homotopy "n"-group" at every point, which will encapsulate the Postnikov tower of the space up to the homotopy group πn, or the entire Postnikov tower for "n" = ∞.