-- Lean 4 proof step from Mathlib.Algebra.Homology.ShortComplex.PreservesHomology (goal)
Goal:
C : Type u_1
D : Type u_2
inst : Category.{v_1, u_1} C
inst_1 : Category.{v_2, u_2} D
inst_2 : HasZeroMorphisms.{v_1, u_1} C
inst_3 : HasZeroMorphisms.{v_2, u_2} D
S₁ S₂ : ShortComplex.{v_1, u_1} C
φ : S₁ ⟶ S₂
F : Functor.{v_1, v_2, u_1, u_2} C D
inst_4 : F.PreservesZeroMorphisms
inst_5 : S₁.HasRightHomology
inst_6 : S₂.HasRightHomology
inst_7 : F.PreservesRightHomologyOf S₁
inst_8 : F.PreservesRightHomologyOf S₂
rw :
  Eq (α := opcycles (S₁.map F) ⟶ opcycles (S₂.map F))
    (CategoryStruct.comp (Y := F.obj S₁.opcycles) (Iso.hom (S₁.mapOpcyclesIso F))
        (CategoryStruct.comp (Y := F.obj S₂.opcycles) (F.map (opcyclesMap φ)) (Iso.inv (S₂.mapOpcyclesIso F))) :
      opcycles (S₁.map F) ⟶ opcycles (S₂.map F))
    (CategoryStruct.comp (Y := F.obj S₁.opcycles) (Iso.hom (S₁.mapOpcyclesIso F))
        (CategoryStruct.comp (Y := opcycles (S₁.map F))
          (Iso.inv (S₁.mapOpcyclesIso F) : F.obj S₁.opcycles ⟶ opcycles (S₁.map F))
          (opcyclesMap (Functor.map F.mapShortComplex φ) :
            opcycles (S₁.map F) ⟶ opcycles (Functor.obj F.mapShortComplex S₂))) :
      opcycles (S₁.map F) ⟶ opcycles (S₂.map F))
rw₁ :
  Eq (α := opcycles (S₁.map F) ⟶ opcycles (S₂.map F))
    (CategoryStruct.comp (Y := opcycles (Functor.obj F.mapShortComplex S₂))
        (opcyclesMap (Functor.map F.mapShortComplex φ) :
          opcycles (Functor.obj F.mapShortComplex S₁) ⟶ opcycles (Functor.obj F.mapShortComplex S₂))
        (CategoryStruct.comp (Y := F.obj S₂.opcycles) (Iso.hom (S₂.mapOpcyclesIso F)) (Iso.inv (S₂.mapOpcyclesIso F)) :
          opcycles (Functor.obj F.mapShortComplex S₂) ⟶ opcycles (S₂.map F)) :
      opcycles (Functor.obj F.mapShortComplex S₁) ⟶ opcycles (S₂.map F))
    (CategoryStruct.comp (Y := F.obj S₁.opcycles) (Iso.hom (S₁.mapOpcyclesIso F))
      (CategoryStruct.comp (Y := opcycles (S₁.map F))
        (Iso.inv (S₁.mapOpcyclesIso F) : F.obj S₁.opcycles ⟶ opcycles (S₁.map F))
        (opcyclesMap (Functor.map F.mapShortComplex φ) :
          opcycles (S₁.map F) ⟶ opcycles (Functor.obj F.mapShortComplex S₂))))
rw₂ :
  Eq (α := opcycles (S₁.map F) ⟶ opcycles (S₂.map F))
    (opcyclesMap (Functor.map F.mapShortComplex φ) ≫ 𝟙 (opcycles (Functor.obj F.mapShortComplex S₂)) :
      opcycles (Functor.obj F.mapShortComplex S₁) ⟶ opcycles (S₂.map F))
    (CategoryStruct.comp (Y := F.obj S₁.opcycles) (Iso.hom (S₁.mapOpcyclesIso F))
        (CategoryStruct.comp (Y := opcycles (S₁.map F))
          (Iso.inv (S₁.mapOpcyclesIso F) : F.obj S₁.opcycles ⟶ opcycles (S₁.map F))
          (opcyclesMap (Functor.map F.mapShortComplex φ) :
            opcycles (S₁.map F) ⟶ opcycles (Functor.obj F.mapShortComplex S₂))) :
      opcycles (S₁.map F) ⟶ opcycles (S₂.map F))
⊢ Eq (α := F.obj S₁.opcycles ⟶ opcycles (S₂.map F))
    (CategoryStruct.comp (Y := F.obj S₂.opcycles) (F.map (opcyclesMap φ)) (Iso.inv (S₂.mapOpcyclesIso F)) :
      F.obj S₁.opcycles ⟶ opcycles (S₂.map F))
    (CategoryStruct.comp (Y := opcycles (S₁.map F))
        (Iso.inv (S₁.mapOpcyclesIso F) : F.obj S₁.opcycles ⟶ opcycles (S₁.map F))
        (opcyclesMap (Functor.map F.mapShortComplex φ) :
          opcycles (S₁.map F) ⟶ opcycles (Functor.obj F.mapShortComplex S₂)) :
      F.obj S₁.opcycles ⟶ opcycles (Functor.obj F.mapShortComplex S₂))
have rw₃ : Eq (α := opcycles (S₁.map F) ⟶ opcycles (S₂.map F))
  (opcyclesMap (Functor.map F.mapShortComplex φ) :
    opcycles (Functor.obj F.mapShortComplex S₁) ⟶ opcycles (Functor.obj F.mapShortComplex S₂))
  (CategoryStruct.comp (Y := F.obj S₁.opcycles) (Iso.hom (S₁.mapOpcyclesIso F))
    (CategoryStruct.comp (Y := opcycles (S₁.map F))
      (Iso.inv (S₁.mapOpcyclesIso F) : F.obj S₁.opcycles ⟶ opcycles (S₁.map F))
      (opcyclesMap (Functor.map F.mapShortComplex φ) :
        opcycles (S₁.map F) ⟶ opcycles (Functor.obj F.mapShortComplex S₂))))
Tactic: rw [← cancel_epi (S₁.mapOpcyclesIso F).hom, ← mapOpcyclesIso_hom_naturality_assoc,
    Iso.hom_inv_id, comp_id, Iso.hom_inv_id_assoc]