fact: Definition empty_zig_zag {C : category} (x : C) : zig_zag x x := 0 ,, identity_z_iso x.
imports: UniMath.Foundations.All UniMath.MoreFoundations.All UniMath.CategoryTheory.Core.Categories UniMath.CategoryTheory.Core.Isos UniMath.CategoryTheory.Core.Functors UniMath.CategoryTheory.Groupoids