`{Univalence} {A B : AbGroup@{u}} (E F G : AbSES B A) : abses_baer_sum (abses_baer_sum E F) G = abses_baer_sum E (abses_baer_sum F G). Proof. refine ((baer_sum_twist _ _ _)^ @ _). refine (baer_sum_commutative _ _ @ _). apply ap. apply baer_sum_commutative. Defined.
Require Import Basics Types. Require Import WildCat Pointed.Core. Require Import AbGroups.AbelianGroup AbGroups.Biproduct AbGroups.AbHom. Require Import AbSES.Core AbSES.Pullback AbSES.Pushout AbSES.DirectSum. Require Import Homotopy.HSpace.Core.