∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] [inst_1 : CategoryTheory.Abelian C]
  [inst_2 : CategoryTheory.HasExt C] {X Y : C} (f : CategoryTheory.Abelian.Ext X Y 0),
  CategoryTheory.Abelian.Ext.mk₀ (CategoryTheory.Abelian.Ext.addEquiv₀ f) = f