-- Lean 4 proof step from Mathlib.Algebra.Homology.SpectralObject.SpectralSequence (goal)
Goal:
C : Type u_1
ι : Type u_2
κ : Type u_3
inst : Category.{v_1, u_1} C
inst_1 : Abelian.{v_1, u_1} C
inst_2 : Preorder ι
X : SpectralObject.{u_1, u_2, v_1, u_2} C ι
c : ℤ → ComplexShape κ
r₀ : ℤ
data : SpectralSequenceDataCore ι c r₀
r r' : ℤ
hrr' : r + 1 = r'
hr : r₀ ≤ r
pq pq' : κ
hpq : (c r).prev pq' = pq
i₀ i₁ i₂ i₃ i₃' : ι
hi₀ : i₀ = data.i₀ r pq' ⋯
hi₁ : i₁ = data.i₁ pq'
hi₂ : i₂ = data.i₂ pq'
hi₃ : i₃ = data.i₃ r pq' ⋯
hi₃' : i₃' = data.i₃ r' pq' ⋯
n₀ n₁ n₂ : ℤ
hn₁' : n₁ = data.deg pq'
inst_3 : X.HasSpectralSequence data
hn₁ : n₀ + 1 = n₁
hn₂ : n₁ + 1 = n₂
h : (c r).Rel pq pq'
⊢ Eq (α := pageX X data r pq ⋯ ⟶ X.E (homOfLE ⋯) (homOfLE ⋯) (homOfLE ⋯) n₀ n₁ n₂ ⋯ ⋯)
    (CategoryStruct.comp (Y :=
      X.E (l := data.i₃ r pq ⋯) (homOfLE ⋯) (homOfLE ⋯) (homOfLE ⋯ : i₃' ⟶ data.i₃ r pq ⋯) (n₀ - 1) n₀ n₁ ⋯ ⋯)
      (Iso.hom (pageXIso X data r hr pq i₂ i₃ i₃' (data.i₃ r pq ⋯) ⋯ ⋯ ⋯ ⋯ (n₀ - 1) n₀ n₁ ⋯ ⋯ ⋯) :
        pageX X data r pq ⋯ ⟶
          X.E (l := data.i₃ r pq ⋯) (homOfLE ⋯) (homOfLE ⋯) (homOfLE ⋯ : i₃' ⟶ data.i₃ r pq ⋯) (n₀ - 1) n₀ n₁ ⋯ ⋯)
      (X.d (homOfLE ⋯) (homOfLE ⋯) (homOfLE ⋯) (homOfLE ⋯) (homOfLE ⋯) (n₀ - 1) n₀ n₁ n₂ ⋯ ⋯ ⋯ :
        X.E (l := data.i₃ r pq ⋯) (homOfLE ⋯) (homOfLE ⋯) (homOfLE ⋯ : i₃' ⟶ data.i₃ r pq ⋯) (n₀ - 1) n₀ n₁ ⋯ ⋯ ⟶
          X.E (homOfLE ⋯) (homOfLE ⋯) (homOfLE ⋯) n₀ n₁ n₂ ⋯ ⋯))
    (pageD X data r pq pq' ⋯ ≫ Iso.hom (pageXIso X data r hr pq' i₀ i₁ i₂ i₃ hi₀ hi₁ hi₂ hi₃ n₀ n₁ n₂ hn₁' ⋯ ⋯))
have pos : Eq (α := pageX X data r pq sorry ⟶ X.E (homOfLE sorry) (homOfLE sorry) (homOfLE sorry) n₀ n₁ n₂ sorry sorry)
  (CategoryStruct.comp (Y :=
    X.E (l := data.i₃ r pq sorry) (homOfLE sorry) (homOfLE sorry) (homOfLE sorry : i₃' ⟶ data.i₃ r pq sorry) (n₀ - 1) n₀ n₁ sorry sorry)
    (Iso.hom (pageXIso X data r hr pq i₂ i₃ i₃' (data.i₃ r pq sorry) sorry sorry sorry sorry (n₀ - 1) n₀ n₁ sorry sorry sorry) :
      pageX X data r pq sorry ⟶
        X.E (l := data.i₃ r pq sorry) (homOfLE sorry) (homOfLE sorry) (homOfLE sorry : i₃' ⟶ data.i₃ r pq sorry) (n₀ - 1) n₀ n₁ sorry sorry)
    (X.d (homOfLE sorry) (homOfLE sorry) (homOfLE sorry) (homOfLE sorry) (homOfLE sorry) (n₀ - 1) n₀ n₁ n₂ sorry sorry sorry :
      X.E (l := data.i₃ r pq sorry) (homOfLE sorry) (homOfLE sorry) (homOfLE sorry : i₃' ⟶ data.i₃ r pq sorry) (n₀ - 1) n₀ n₁ sorry sorry ⟶
        X.E (homOfLE sorry) (homOfLE sorry) (homOfLE sorry) n₀ n₁ n₂ sorry sorry))
  (CategoryStruct.comp (Y :=
      X.E (l := data.i₃ r pq sorry) (homOfLE sorry) (homOfLE sorry) (homOfLE sorry : i₃' ⟶ data.i₃ r pq sorry) (n₀ - 1) n₀ n₁ sorry sorry)
      (Iso.hom (pageXIso X data r sorry pq i₂ i₃ i₃' (data.i₃ r pq sorry) sorry sorry sorry sorry (n₀ - 1) n₀ n₁ sorry sorry sorry) :
        pageX X data r pq sorry ⟶
          X.E (l := data.i₃ r pq sorry) (homOfLE sorry) (homOfLE sorry) (homOfLE sorry : i₃' ⟶ data.i₃ r pq sorry) (n₀ - 1) n₀ n₁ sorry sorry)
      (CategoryStruct.comp (Y := X.E (homOfLE sorry) (homOfLE sorry) (homOfLE sorry) n₀ n₁ n₂ sorry sorry)
          (X.d (homOfLE sorry) (homOfLE sorry) (homOfLE sorry) (homOfLE sorry) (homOfLE sorry) (n₀ - 1) n₀ n₁ n₂ sorry sorry sorry)
          (Iso.inv (pageXIso X data r sorry pq' i₀ i₁ i₂ i₃ hi₀ hi₁ sorry sorry n₀ n₁ n₂ sorry sorry sorry)) :
        X.E (l := data.i₃ r pq sorry) (homOfLE sorry) (homOfLE sorry) (homOfLE sorry : i₃' ⟶ data.i₃ r pq sorry) (n₀ - 1) n₀ n₁ sorry sorry ⟶
          pageX X data r pq' sorry) ≫
    Iso.hom (pageXIso X data r hr pq' i₀ i₁ i₂ i₃ hi₀ hi₁ hi₂ hi₃ n₀ n₁ n₂ hn₁' sorry sorry))
Tactic: rw [pageD_eq X data r hr pq pq' h
          (homOfLE (data.le₀₁' r hr pq' hi₀ hi₁)) (homOfLE (data.le₁₂' pq' hi₁ hi₂))
          (homOfLE (data.le₂₃' r hr pq' hi₂ hi₃)) (homOfLE (data.le₃₃' hrr' hr pq' hi₃ hi₃'))
          (homOfLE (data.le₂₃' r hr pq (by rw [hi₃', data.i₃_next r r' pq pq' h]) rfl))
          hi₀ hi₁ (hi₂.trans (data.hc₀₂ r pq pq' h).symm)
          (hi₃.trans (data.hc₁₃ r pq pq' h).symm) (hi₃'.trans (data.i₃_next r r' pq pq' h)) rfl
          (n₀ - 1) n₀ n₁ n₂ (by have := data.hc r _ _ h; lia),
        Category.assoc, Category.assoc, Iso.inv_hom_id, Category.comp_id]