∀ {C : Type u_2} [inst : CategoryTheory.Category.{u_1, u_2} C] [inst_1 : CategoryTheory.Preadditive C]
  (K L : CochainComplex C ℤ) (n m : ℤ) (hm : n + 1 = m) (s : CategoryTheory.Limits.Fork ((K.HomComplex L).d n m) 0)
  (x y : ↑s.1),
  ⟨(CategoryTheory.ConcreteCategory.hom s.ι) (x + y), ⋯⟩ =
    ⟨(CategoryTheory.ConcreteCategory.hom s.ι) x, ⋯⟩ + ⟨(CategoryTheory.ConcreteCategory.hom s.ι) y, ⋯⟩