fact: Definition pi2_s2 `{Univalence} : Pi 2 (psphere 2) $<~> abgroup_Z.
imports: Basics Types WildCat Pointed Truncations.Core Truncations.Connectedness Spaces.Int Spaces.Circle Spaces.Spheres Algebra.AbGroups Homotopy.HomotopyGroup Homotopy.HSpaceS1 Homotopy.Hopf