∀ {R : Type u_3} {C : Type u_2} [inst : Semiring R] [inst_1 : CategoryTheory.Category.{u_1, u_2} C]
  [inst_2 : CategoryTheory.Preadditive C] [inst_3 : CategoryTheory.Linear R C] {S₁ S₂ : CategoryTheory.ShortComplex C}
  (b : S₁ ⟶ S₂), 1 • b = b