-- Lean 4 proof step from Mathlib.CategoryTheory.Abelian.RightDerived (goal)
Goal:
C : Type u
inst : Category.{v, u} C
D : Type u_1
inst_1 : Category.{v_1, u_1} D
inst_2 : Abelian.{v, u} C
inst_3 : HasInjectiveResolutions.{v, u} C
inst_4 : Abelian.{v_1, u_1} D
X Y : C
f : X ⟶ Y
I : InjectiveResolution.{v, u} X
J : InjectiveResolution.{v, u} Y
φ : I.cocomplex ⟶ J.cocomplex
comm :
  Eq (α := HomologicalComplex.X.{v, u, 0} (Functor.obj (CochainComplex.single₀ C) X) 0 ⟶ J.cocomplex.X 0)
    (I.ι.f 0 ≫ φ.f 0) (f ≫ J.ι.f 0)
F : Functor.{v, v_1, u, u_1} C D
inst_5 : F.Additive
n : ℕ
rw :
  (Iso.inv (I.isoRightDerivedObj F n) ≫ (F.rightDerived n).map f) ≫ Iso.hom (J.isoRightDerivedObj F n) =
    CategoryStruct.comp (Y := (F.rightDerived n).obj Y)
      (Functor.map
          (Functor.comp (D := HomologicalComplex.{v_1, u_1, 0} D (ComplexShape.up ℕ))
            (F.mapHomologicalComplex (ComplexShape.up ℕ)) (HomologicalComplex.homologyFunctor D (ComplexShape.up ℕ) n))
          φ ≫
        Iso.inv (J.isoRightDerivedObj F n))
      (Iso.hom (J.isoRightDerivedObj F n))
rw₁ :
  Iso.inv (I.isoRightDerivedObj F n) ≫ (F.rightDerived n).map f ≫ Iso.hom (J.isoRightDerivedObj F n) =
    CategoryStruct.comp (Y := (F.rightDerived n).obj Y)
      (Functor.map
          (Functor.comp (D := HomologicalComplex.{v_1, u_1, 0} D (ComplexShape.up ℕ))
            (F.mapHomologicalComplex (ComplexShape.up ℕ)) (HomologicalComplex.homologyFunctor D (ComplexShape.up ℕ) n))
          φ ≫
        Iso.inv (J.isoRightDerivedObj F n))
      (Iso.hom (J.isoRightDerivedObj F n))
rw₂ :
  Iso.inv (I.isoRightDerivedObj F n) ≫ (F.rightDerived n).map f ≫ Iso.hom (J.isoRightDerivedObj F n) =
    CategoryStruct.comp (Y :=
      Functor.obj
        (Functor.comp (D := HomologicalComplex.{v_1, u_1, 0} D (ComplexShape.up ℕ))
          (F.mapHomologicalComplex (ComplexShape.up ℕ)) (HomologicalComplex.homologyFunctor D (ComplexShape.up ℕ) n))
        J.cocomplex)
      (Functor.map
          (Functor.comp (D := HomologicalComplex.{v_1, u_1, 0} D (ComplexShape.up ℕ))
            (F.mapHomologicalComplex (ComplexShape.up ℕ)) (HomologicalComplex.homologyFunctor D (ComplexShape.up ℕ) n))
          φ :
        Functor.obj
            (Functor.comp (D := HomologicalComplex.{v_1, u_1, 0} D (ComplexShape.up ℕ))
              (F.mapHomologicalComplex (ComplexShape.up ℕ))
              (HomologicalComplex.homologyFunctor D (ComplexShape.up ℕ) n))
            I.cocomplex ⟶
          Functor.obj
            (Functor.comp (D := HomologicalComplex.{v_1, u_1, 0} D (ComplexShape.up ℕ))
              (F.mapHomologicalComplex (ComplexShape.up ℕ))
              (HomologicalComplex.homologyFunctor D (ComplexShape.up ℕ) n))
            J.cocomplex)
      (CategoryStruct.comp (Y := (F.rightDerived n).obj Y) (Iso.inv (J.isoRightDerivedObj F n))
          (Iso.hom (J.isoRightDerivedObj F n)) :
        Functor.obj
            (Functor.comp (D := HomologicalComplex.{v_1, u_1, 0} D (ComplexShape.up ℕ))
              (F.mapHomologicalComplex (ComplexShape.up ℕ))
              (HomologicalComplex.homologyFunctor D (ComplexShape.up ℕ) n))
            J.cocomplex ⟶
          Functor.obj (C := HomologicalComplex.{v_1, u_1, 0} D (ComplexShape.up ℕ))
            (HomologicalComplex.homologyFunctor D (ComplexShape.up ℕ) n)
            (Functor.obj (F.mapHomologicalComplex (ComplexShape.up ℕ)) J.cocomplex))
⊢ Iso.inv (I.isoRightDerivedObj F n) ≫ (F.rightDerived n).map f =
    Functor.map
        (Functor.comp (D := HomologicalComplex.{v_1, u_1, 0} D (ComplexShape.up ℕ))
          (F.mapHomologicalComplex (ComplexShape.up ℕ)) (HomologicalComplex.homologyFunctor D (ComplexShape.up ℕ) n))
        φ ≫
      Iso.inv (J.isoRightDerivedObj F n)
have rw₃ : Eq (α :=
  Functor.obj (C := HomologicalComplex.{v_1, u_1, 0} D (ComplexShape.up ℕ))
      (HomologicalComplex.homologyFunctor D (ComplexShape.up ℕ) n)
      (Functor.obj (F.mapHomologicalComplex (ComplexShape.up ℕ)) I.cocomplex) ⟶
    Functor.obj (C := HomologicalComplex.{v_1, u_1, 0} D (ComplexShape.up ℕ))
      (HomologicalComplex.homologyFunctor D (ComplexShape.up ℕ) n)
      (Functor.obj (F.mapHomologicalComplex (ComplexShape.up ℕ)) J.cocomplex))
  (CategoryStruct.comp (Y := (F.rightDerived n).obj X) (Iso.inv (I.isoRightDerivedObj F n))
      (Iso.hom (I.isoRightDerivedObj F n) ≫
        Functor.map
          (Functor.comp (D := HomologicalComplex.{v_1, u_1, 0} D (ComplexShape.up ℕ))
            (F.mapHomologicalComplex (ComplexShape.up ℕ)) (HomologicalComplex.homologyFunctor D (ComplexShape.up ℕ) n))
          φ) :
    Functor.obj (C := HomologicalComplex.{v_1, u_1, 0} D (ComplexShape.up ℕ))
        (HomologicalComplex.homologyFunctor D (ComplexShape.up ℕ) n)
        (Functor.obj (F.mapHomologicalComplex (ComplexShape.up ℕ)) I.cocomplex) ⟶
      Functor.obj (C := HomologicalComplex.{v_1, u_1, 0} D (ComplexShape.up ℕ))
        (HomologicalComplex.homologyFunctor D (ComplexShape.up ℕ) n)
        (Functor.obj (F.mapHomologicalComplex (ComplexShape.up ℕ)) J.cocomplex))
  (CategoryStruct.comp (Y :=
      Functor.obj
        (Functor.comp (D := HomologicalComplex.{v_1, u_1, 0} D (ComplexShape.up ℕ))
          (F.mapHomologicalComplex (ComplexShape.up ℕ)) (HomologicalComplex.homologyFunctor D (ComplexShape.up ℕ) n))
        J.cocomplex)
      (Functor.map
          (Functor.comp (D := HomologicalComplex.{v_1, u_1, 0} D (ComplexShape.up ℕ))
            (F.mapHomologicalComplex (ComplexShape.up ℕ)) (HomologicalComplex.homologyFunctor D (ComplexShape.up ℕ) n))
          φ :
        Functor.obj
            (Functor.comp (D := HomologicalComplex.{v_1, u_1, 0} D (ComplexShape.up ℕ))
              (F.mapHomologicalComplex (ComplexShape.up ℕ))
              (HomologicalComplex.homologyFunctor D (ComplexShape.up ℕ) n))
            I.cocomplex ⟶
          Functor.obj
            (Functor.comp (D := HomologicalComplex.{v_1, u_1, 0} D (ComplexShape.up ℕ))
              (F.mapHomologicalComplex (ComplexShape.up ℕ))
              (HomologicalComplex.homologyFunctor D (ComplexShape.up ℕ) n))
            J.cocomplex)
      (CategoryStruct.comp (Y := (F.rightDerived n).obj Y) (Iso.inv (J.isoRightDerivedObj F n))
          (Iso.hom (J.isoRightDerivedObj F n)) :
        Functor.obj
            (Functor.comp (D := HomologicalComplex.{v_1, u_1, 0} D (ComplexShape.up ℕ))
              (F.mapHomologicalComplex (ComplexShape.up ℕ))
              (HomologicalComplex.homologyFunctor D (ComplexShape.up ℕ) n))
            J.cocomplex ⟶
          Functor.obj (C := HomologicalComplex.{v_1, u_1, 0} D (ComplexShape.up ℕ))
            (HomologicalComplex.homologyFunctor D (ComplexShape.up ℕ) n)
            (Functor.obj (F.mapHomologicalComplex (ComplexShape.up ℕ)) J.cocomplex)) :
    Functor.obj (C := HomologicalComplex.{v_1, u_1, 0} D (ComplexShape.up ℕ))
        (HomologicalComplex.homologyFunctor D (ComplexShape.up ℕ) n)
        (Functor.obj (F.mapHomologicalComplex (ComplexShape.up ℕ)) I.cocomplex) ⟶
      Functor.obj (C := HomologicalComplex.{v_1, u_1, 0} D (ComplexShape.up ℕ))
        (HomologicalComplex.homologyFunctor D (ComplexShape.up ℕ) n)
        (Functor.obj (F.mapHomologicalComplex (ComplexShape.up ℕ)) J.cocomplex))
Tactic: rw [← cancel_mono (J.isoRightDerivedObj F n).hom, assoc, assoc,
    InjectiveResolution.isoRightDerivedObj_hom_naturality f I J φ comm F n,
    Iso.inv_hom_id_assoc, Iso.inv_hom_id, comp_id]