∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] [inst_1 : CategoryTheory.Abelian C]
  [inst_2 : CategoryTheory.HasExt C] {X Y : C} {n : ℕ} (α : CategoryTheory.Abelian.Ext X Y n),
  α.comp (CategoryTheory.Abelian.Ext.mk₀ (CategoryTheory.CategoryStruct.id Y)) ⋯ = α