-- Lean 4 proof step from Mathlib.Algebra.Homology.Homotopy (goal)
Goal:
ι : Type u_1
V : Type u
inst : Category.{v, u} V
inst_1 : Preadditive.{v, u} V
c : ComplexShape ι
C D E : HomologicalComplex.{v, u, u_1} V c
f g : C ⟶ D
h k : D ⟶ E
i_1 : ι
P Q : ChainComplex.{v, u, 0} V ℕ
e : P ⟶ Q
zero : P.X 0 ⟶ Q.X 1
comm_zero : e.f 0 = zero ≫ Q.d 1 0
one : P.X 1 ⟶ Q.X 2
comm_one : e.f 1 = P.d 1 0 ≫ zero + one ≫ Q.d 2 1
succ :
  (n : ℕ) →
    (p :
        (f : P.X n ⟶ Q.X (n + 1)) ×'
          (f' : P.X (n + 1) ⟶ Q.X (n + 2)) ×' e.f (n + 1) = P.d (n + 1) n ≫ f + f' ≫ Q.d (n + 2) (n + 1)) →
      (f'' : P.X (n + 2) ⟶ Q.X (n + 3)) ×'
        e.f (n + 2) =
          P.d (n + 2) (n + 1) ≫
              p.snd (β := fun (f : P.X n ⟶ Q.X (n + 1)) ↦
                    (f' : P.X (n + 1) ⟶ Q.X (n + 2)) ×' e.f (n + 1) = P.d (n + 1) n ≫ f + f' ≫ Q.d (n + 2) (n + 1)).fst
                (β := fun (f' : P.X (n + 1) ⟶ Q.X (n + 2)) ↦
                e.f (n + 1) =
                  P.d (n + 1) n ≫
                      p.fst (β := fun (f : P.X n ⟶ Q.X (n + 1)) ↦
                        (f' : P.X (n + 1) ⟶ Q.X (n + 2)) ×'
                          e.f (n + 1) = P.d (n + 1) n ≫ f + f' ≫ Q.d (n + 2) (n + 1)) +
                    f' ≫ Q.d (n + 2) (n + 1)) +
            f'' ≫ Q.d (n + 3) (n + 2)
i : ℕ
⊢ (mkInductiveAux₂ e zero comm_zero one comm_one succ i).snd (β := fun (f : xNext P i ⟶ Q.X i) ↦
          (f' : P.X i ⟶ xPrev Q i) ×' e.f i = dFrom P i ≫ f + f' ≫ dTo Q i).fst
      (β := fun (f' : P.X i ⟶ xPrev Q i) ↦
      e.f i =
        dFrom P i ≫
            (mkInductiveAux₂ e zero comm_zero one comm_one succ i).fst (β := fun (f : xNext P i ⟶ Q.X i) ↦
              (f' : P.X i ⟶ xPrev Q i) ×' e.f i = dFrom P i ≫ f + f' ≫ dTo Q i) +
          f' ≫ dTo Q i) =
    if h : i + 1 = (ComplexShape.down ℕ).prev i then
      (mkInductiveAux₂ e zero comm_zero one comm_one succ i).snd (β := fun (f : xNext P i ⟶ Q.X i) ↦
              (f' : P.X i ⟶ xPrev Q i) ×' e.f i = dFrom P i ≫ f + f' ≫ dTo Q i).fst
          (β := fun (f' : P.X i ⟶ xPrev Q i) ↦
          e.f i =
            dFrom P i ≫
                (mkInductiveAux₂ e zero comm_zero one comm_one succ i).fst (β := fun (f : xNext P i ⟶ Q.X i) ↦
                  (f' : P.X i ⟶ xPrev Q i) ×' e.f i = dFrom P i ≫ f + f' ≫ dTo Q i) +
              f' ≫ dTo Q i) ≫
        (xPrevIso Q h).hom
    else 0
have e_a : (mkInductiveAux₂ e zero comm_zero one comm_one succ i).snd (β := fun (f : xNext P i ⟶ Q.X i) ↦
        (f' : P.X i ⟶ xPrev Q i) ×' e.f i = dFrom P i ≫ f + f' ≫ dTo Q i).fst
    (β := fun (f' : P.X i ⟶ xPrev Q i) ↦
    e.f i =
      dFrom P i ≫
          (mkInductiveAux₂ e zero comm_zero one comm_one succ i).fst (β := fun (f : xNext P i ⟶ Q.X i) ↦
            (f' : P.X i ⟶ xPrev Q i) ×' e.f i = dFrom P i ≫ f + f' ≫ dTo Q i) +
        f' ≫ dTo Q i) =
  (mkInductiveAux₂ e zero comm_zero one comm_one succ i).snd (β := fun (f : xNext P i ⟶ Q.X i) ↦
          (f' : P.X i ⟶ xPrev Q i) ×' e.f i = dFrom P i ≫ f + f' ≫ dTo Q i).fst
      (β := fun (f' : P.X i ⟶ xPrev Q i) ↦
      e.f i =
        dFrom P i ≫
            (mkInductiveAux₂ e zero comm_zero one comm_one succ i).fst (β := fun (f : xNext P i ⟶ Q.X i) ↦
              (f' : P.X i ⟶ xPrev Q i) ×' e.f i = dFrom P i ≫ f + f' ≫ dTo Q i) +
          f' ≫ dTo Q i) ≫
    (xPrevIso Q sorry).hom
Tactic: rw [dif_pos (by simp only [ChainComplex.prev])]