Lean4
/-- the sum of two homotopies is a homotopy between the sum of the respective morphisms. -/
@[simps!]
def add {f₁ g₁ f₂ g₂ : C ⟶ D} (h₁ : Homotopy f₁ g₁) (h₂ : Homotopy f₂ g₂) : Homotopy (f₁ + f₂) (g₁ + g₂)
where
hom := h₁.hom + h₂.hom
zero i j hij := by rw [Pi.add_apply, Pi.add_apply, h₁.zero i j hij, h₂.zero i j hij, add_zero]
comm
i := by
simp only [HomologicalComplex.add_f_apply, h₁.comm, h₂.comm, AddMonoidHom.map_add]
abel