∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] [inst_1 : CategoryTheory.Abelian C]
  [inst_2 : CategoryTheory.HasExt C] {X Y : C} (R : CategoryTheory.ProjectiveResolution X) {n : ℕ}
  (f : R.complex.X n ⟶ Y) (m : ℕ) (hm : n + 1 = m) (hf : CategoryTheory.CategoryStruct.comp (R.complex.d m n) f = 0)
  {Y' : C} (g : Y ⟶ Y'),
  (R.extMk f m hm hf).comp (CategoryTheory.Abelian.Ext.mk₀ g) ⋯ =
    R.extMk (CategoryTheory.CategoryStruct.comp f g) m hm ⋯