∀ {C : Type u_3} [inst : CategoryTheory.Category.{u_2, u_3} C] [inst_1 : CategoryTheory.Abelian C]
  [inst_2 : CategoryTheory.HasExt C] {X Y : C},
  Function.LeftInverse CategoryTheory.Abelian.Ext.addEquiv₀.invFun CategoryTheory.Abelian.Ext.addEquiv₀.toFun