∀ {C : Type u_1} [inst : CategoryTheory.Category.{v_1, u_1} C] [inst_1 : CategoryTheory.Abelian C]
  {S₁ S₂ S₃ : CategoryTheory.ShortComplex.SnakeInput C} (f : S₁ ⟶ S₂) (g : S₂ ⟶ S₃) {Z : CategoryTheory.ShortComplex C}
  (h : S₃.L₁ ⟶ Z),
  CategoryTheory.CategoryStruct.comp (CategoryTheory.CategoryStruct.comp f g).f₁ h =
    CategoryTheory.CategoryStruct.comp f.f₁ (CategoryTheory.CategoryStruct.comp g.f₁ h)