-- Lean 4 proof step from Mathlib.MeasureTheory.SetAlgebra (goal)
Goal:
α : Type u_1
𝒜 : Set (Set α)
s : Set α
s_mem : s ∈ generateSetAlgebra 𝒜
base :
  ∀ (u : Set α) (u_mem : u ∈ 𝒜),
    ∃ (A : Set (Set (Set α))),
      A.Finite ∧ (∀ a ∈ A, a.Finite) ∧ (∀ a ∈ A, ∀ t ∈ a, t ∈ 𝒜 ∨ tᶜ ∈ 𝒜) ∧ u = ⋃ a ∈ A, ⋂ t ∈ a, t
empty :
  ∃ (A : Set (Set (Set α))),
    A.Finite ∧
      (∀ a ∈ A, a.Finite) ∧ (∀ a ∈ A, ∀ t ∈ a, t ∈ 𝒜 ∨ tᶜ ∈ 𝒜) ∧ Eq.{u_1 + 1} (α := Set α) ∅ (⋃ a ∈ A, ⋂ t ∈ a, t)
compl :
  ∀ (u : Set α),
    generateSetAlgebra 𝒜 u →
      ∀
        (u_ind :
          ∃ (A : Set (Set (Set α))),
            A.Finite ∧ (∀ a ∈ A, a.Finite) ∧ (∀ a ∈ A, ∀ t ∈ a, t ∈ 𝒜 ∨ tᶜ ∈ 𝒜) ∧ u = ⋃ a ∈ A, ⋂ t ∈ a, t),
        ∃ (A : Set (Set (Set α))),
          A.Finite ∧ (∀ a ∈ A, a.Finite) ∧ (∀ a ∈ A, ∀ t ∈ a, t ∈ 𝒜 ∨ tᶜ ∈ 𝒜) ∧ uᶜ = ⋃ a ∈ A, ⋂ t ∈ a, t
⊢ ∃ (A : Set (Set (Set α))),
    A.Finite ∧ (∀ a ∈ A, a.Finite) ∧ (∀ a ∈ A, ∀ t ∈ a, t ∈ 𝒜 ∨ tᶜ ∈ 𝒜) ∧ s = ⋃ a ∈ A, ⋂ t ∈ a, t
have union : ∀ (u v : Set α),
  generateSetAlgebra 𝒜 u →
    generateSetAlgebra 𝒜 v →
      ∀
        (u_ind :
          ∃ (A : Set (Set (Set α))),
            A.Finite ∧ (∀ a ∈ A, a.Finite) ∧ (∀ a ∈ A, ∀ t ∈ a, t ∈ 𝒜 ∨ tᶜ ∈ 𝒜) ∧ u = ⋃ a ∈ A, ⋂ t ∈ a, t)
        (v_ind :
          ∃ (A : Set (Set (Set α))),
            A.Finite ∧ (∀ a ∈ A, a.Finite) ∧ (∀ a ∈ A, ∀ t ∈ a, t ∈ 𝒜 ∨ tᶜ ∈ 𝒜) ∧ v = ⋃ a ∈ A, ⋂ t ∈ a, t),
        ∃ (A : Set (Set (Set α))),
          A.Finite ∧ (∀ a ∈ A, a.Finite) ∧ (∀ a ∈ A, ∀ t ∈ a, t ∈ 𝒜 ∨ tᶜ ∈ 𝒜) ∧ u ∪ v = ⋃ a ∈ A, ⋂ t ∈ a, t
Tactic: induction s_mem with
  | base u u_mem =>
    refine ⟨{{u}}, finite_singleton {u},
      fun a ha ↦ eq_of_mem_singleton ha ▸ finite_singleton u,
      fun a ha t ht ↦ ?_, by simp⟩
    rw [eq_of_mem_singleton ha, ha, eq_of_mem_singleton ht, ht] at *
    exact Or.inl u_mem
  | empty => exact ⟨∅, finite_empty, fun _ h ↦ (notMem_empty _ h).elim,
    fun _ ha _ _ ↦ (notMem_empty _ ha).elim, by simp⟩
  | compl u _ u_ind =>
    rcases u_ind with ⟨A, A_fin, mem_A, hA, u_eq⟩
    have := finite_coe_iff.2 A_fin
    have := fun a : A ↦ finite_coe_iff.2 <| mem_A a.1 a.2
    refine ⟨{{(f a).1ᶜ | a : A} | f : (Π a : A, ↑a)}, finite_coe_iff.1 inferInstance,
      fun a ⟨f, hf⟩ ↦ hf ▸ finite_coe_iff.1 inferInstance, fun a ha t ht ↦ ?_, ?_⟩
    · rcases ha with ⟨f, rfl⟩
      rcases ht with ⟨a, rfl⟩
      rw [compl_compl, or_comm]
      exact hA a.1 a.2 (f a).1 (f a).2
    · ext x
      simp only [u_eq, compl_iUnion, compl_iInter, mem_iInter, mem_iUnion, mem_compl_iff,
        exists_prop, Subtype.exists, mem_setOf_eq, iUnion_exists, iUnion_iUnion_eq',
        iInter_exists]
      constructor <;> intro hx
      · choose f hf using hx
        exact ⟨fun ⟨a, ha⟩ ↦ ⟨f a ha, (hf a ha).1⟩, fun _ a ha h ↦ by rw [← h]; exact (hf a ha).2⟩
      · rcases hx with ⟨f, hf⟩
        exact fun a ha ↦ ⟨f ⟨a, ha⟩, (f ⟨a, ha⟩).2, hf (f ⟨a, ha⟩)ᶜ a ha rfl⟩
  | union u v _ _ u_ind v_ind =>
    rcases u_ind with ⟨Au, Au_fin, mem_Au, hAu, u_eq⟩
    rcases v_ind with ⟨Av, Av_fin, mem_Av, hAv, v_eq⟩
    refine ⟨Au ∪ Av, Au_fin.union Av_fin, ?_, ?_, by rw [u_eq, v_eq, ← biUnion_union]⟩
    · rintro a (ha | ha)
      · exact mem_Au a ha
      · exact mem_Av a ha
    · rintro a (ha | ha) t ht
      · exact hAu a ha t ht
      · exact hAv a ha t ht
Cleared hypotheses: s_mem