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