-- Lean 4 proof step from Mathlib.CategoryTheory.Triangulated.Adjunction (hypothesis)
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 : HasZeroObject.{v_1, u_1} C
inst_3 : HasZeroObject.{v_2, u_2} D
inst_4 : Preadditive.{v_1, u_1} C
inst_5 : Preadditive.{v_2, u_2} D
inst_6 : HasShift.{v_1, u_1, 0} C ℤ
inst_7 : HasShift.{v_2, u_2, 0} D ℤ
inst_8 : ∀ (n : ℤ), Functor.Additive.{v_1, u_1, v_1, u_1} (shiftFunctor C n)
inst_9 : ∀ (n : ℤ), Functor.Additive.{v_2, u_2, v_2, u_2} (shiftFunctor D n)
inst_10 : Pretriangulated.{v_1, u_1} C
inst_11 : Pretriangulated.{v_2, u_2} D
F : Functor.{v_1, v_2, u_1, u_2} C D
G : Functor.{v_2, v_1, u_2, u_1} D C
adj : F ⊣ G
inst_12 : F.CommShift ℤ
inst_13 : G.CommShift ℤ
inst_14 : adj.CommShift ℤ
inst_15 : F.IsTriangulated
T : Triangle.{v_2, u_2} D
hT : Membership.mem (γ := Set (Triangle.{v_2, u_2} D)) (@distinguishedTriangles _ _ inst_3 _ inst_5 inst_9 _) T
this_1 : G.Additive
Z : C
f : G.obj T.obj₂ ⟶ Z
g : Z ⟶ Functor.obj.{v_1, v_1, u_1, u_1} (shiftFunctor C (A := ℤ) 1) (G.obj T.obj₁)
mem :
  Membership.mem (γ := Set (Triangle.{v_1, u_1} C)) (@distinguishedTriangles _ _ inst_2 _ inst_4 inst_8 _)
    (Triangle.mk (G.map T.mor₁) f g)
h : F.obj Z ⟶ T.obj₃
h₁ : F.map f ≫ h = adj.counit.app T.obj₂ ≫ T.mor₂
h₂ :
  F.map g ≫
      NatTrans.app (Iso.hom (Functor.commShiftIso F 1)) (G.obj T.obj₁) ≫
        Functor.map (shiftFunctor D (A := ℤ) 1) (adj.counit.app T.obj₁) =
    h ≫ T.mor₃
h₁' : f ≫ adj.unit.app Z ≫ G.map h = G.map T.mor₂
h₂' :
  Eq (α := Z ⟶ (shiftFunctor D (A := ℤ) 1 ⋙ G).obj T.obj₁)
    (g ≫ NatTrans.app (Iso.inv (Functor.commShiftIso G 1)) T.obj₁)
    (((adj.homEquiv Z T.obj₃ : Equiv.{v_2 + 1, v_1 + 1} (F.obj Z ⟶ T.obj₃) (Z ⟶ G.obj T.obj₃)) :
          (a : F.obj Z ⟶ T.obj₃) → Z ⟶ G.obj T.obj₃)
        h ≫
      G.map T.mor₃)
this :
  Mono.{v_1, u_1} (X := Z)
    (((adj.homEquiv Z T.obj₃ : Equiv.{v_2 + 1, v_1 + 1} (F.obj Z ⟶ T.obj₃) (Z ⟶ G.obj T.obj₃)) :
          (a : F.obj Z ⟶ T.obj₃) → Z ⟶ G.obj T.obj₃)
        h :
      Z ⟶ G.obj T.obj₃)
⊢ Membership.mem (γ := Set (Triangle.{v_1, u_1} C)) (@distinguishedTriangles _ _ inst_2 _ inst_4 inst_8 _)
    (Triangle.mk (G.map T.mor₁) (G.map T.mor₂)
      (G.map T.mor₃ ≫ NatTrans.app (Iso.hom (Functor.commShiftIso G 1)) T.obj₁))
have this₁ : IsIso.{v_1, u_1} (X := Z)
  (((adj.homEquiv Z T.obj₃ : Equiv.{v_2 + 1, v_1 + 1} (F.obj Z ⟶ T.obj₃) (Z ⟶ G.obj T.obj₃)) :
        (a : F.obj Z ⟶ T.obj₃) → Z ⟶ G.obj T.obj₃)
      h :
    Z ⟶ G.obj T.obj₃)
Tactic: have := isIso_of_yoneda_map_bijective (adj.homEquiv _ _ h) (fun Y => by
      constructor
      · intro φ₁ φ₂ hφ
        rw [← cancel_mono (adj.homEquiv _ _ h)]
        exact hφ
      · intro φ
        obtain ⟨ψ, hψ⟩ := Triangle.coyoneda_exact₁ _ mem (φ ≫ G.map T.mor₃ ≫
          (G.commShiftIso (1 : ℤ)).hom.app T.obj₁) (by
            dsimp
            rw [assoc, assoc, ← G.commShiftIso_hom_naturality, ← G.map_comp_assoc,
              comp_distTriang_mor_zero₃₁ _ hT, G.map_zero, zero_comp, comp_zero])
        dsimp at ψ hψ
        obtain ⟨α, hα⟩ : ∃ α, α = φ - ψ ≫ adj.homEquiv _ _ h := ⟨_, rfl⟩
        have hα₀ : α ≫ G.map T.mor₃ = 0 := by
          rw [hα, sub_comp, ← cancel_mono ((Functor.commShiftIso G (1 : ℤ)).hom.app T.obj₁),
            assoc, sub_comp, assoc, assoc, hψ, zero_comp, sub_eq_zero,
            ← cancel_mono ((Functor.commShiftIso G (1 : ℤ)).inv.app T.obj₁), assoc,
            assoc, assoc, assoc, h₂', Iso.hom_inv_id_app, comp_id]
        suffices ∃ (β : Y ⟶ Z), β ≫ adj.homEquiv _ _ h = α by
          obtain ⟨β, hβ⟩ := this
          refine ⟨ψ + β, ?_⟩
          dsimp
          rw [add_comp, hβ, hα, add_sub_cancel]
        obtain ⟨β, hβ⟩ := T.coyoneda_exact₃ hT ((adj.homEquiv _ _).symm α)
          ((adj.homEquiv _ _).injective (by simpa [homEquiv_unit, homEquiv_counit] using hα₀))
        refine ⟨adj.homEquiv _ _ β ≫ f, ?_⟩
        simpa [homEquiv_unit, h₁'] using congr_arg (adj.homEquiv _ _).toFun hβ.symm)