/-- Prime avoidance. Atiyah-Macdonald 1.11, Eisenbud 3.3, Stacks 00DS, Matsumura Ex.1.6. -/
theorem subset_union_prime {R : Type u} [CommRing R] {s : Finset ι} {f : ι → Ideal R} (a b : ι)
    (hp : ∀ i ∈ s, i ≠ a → i ≠ b → IsPrime (f i)) {I : Ideal R} :
    ((I : Set R) ⊆ ⋃ i ∈ (↑s : Set ι), f i) ↔ ∃ i ∈ s, I ≤ f i :=
  suffices ((I : Set R) ⊆ ⋃ i ∈ (↑s : Set ι), f i) → ∃ i, i ∈ s ∧ I ≤ f i by
    have aux := fun h => (bex_def.2 <| this h)
    simp_rw [exists_prop] at aux
    refine ⟨aux, fun ⟨i, his, hi⟩ ↦ Set.Subset.trans hi ?_⟩
    apply Set.subset_biUnion_of_mem (show i ∈ (↑s : Set ι) from his)
  fun h : (I : Set R) ⊆ ⋃ i ∈ (↑s : Set ι), f i => by
  

Proof state:
R✝ : Type u
ι : Type u_1
inst✝¹ : CommSemiring R✝
I✝ J K L : Ideal R✝
R : Type u
inst✝ : CommRing R
s : Finset ι
f : ι → Ideal R
a b : ι
hp : ∀ i ∈ s, i ≠ a → i ≠ b → IsPrime (f i)
I : Ideal R
h : ↑I ⊆ ⋃ i ∈ ↑s, ↑(f i)
⊢ ∃ i ∈ s, I ≤ f i

Next tactic:
classical
    by_cases has : a ∈ s
    · obtain ⟨t, hat, rfl⟩ : ∃ t, a ∉ t ∧ insert a t = s :=
        ⟨s.erase a, Finset.not_mem_erase a s, Finset.insert_erase has⟩
      by_cases hbt : b ∈ t
      · obtain ⟨u, hbu, rfl⟩ : ∃ u, b ∉ u ∧ insert b u = t :=
          ⟨t.erase b, Finset.not_mem_erase b t, Finset.insert_erase hbt⟩
        have hp' : ∀ i ∈ u, IsPrime (f i) := by
          intro i hiu
          refine' hp i (Finset.mem_insert_of_mem (Finset.mem_insert_of_mem hiu)) _ _ <;>
              rintro rfl <;>
            solve_by_elim only [Finset.mem_insert_of_mem, *]
        rw [Finset.coe_insert, Finset.coe_insert, Set.biUnion_insert, Set.biUnion_insert, ←
          Set.union_assoc, subset_union_prime' hp'] at h
        rwa [Finset.exists_mem_insert, Finset.exists_mem_insert]
      · have hp' : ∀ j ∈ t, IsPrime (f j) := by
          intro j hj
          refine' hp j (Finset.mem_insert_of_mem hj) _ _ <;> rintro rfl <;>
            solve_by_elim only [Finset.mem_insert_of_mem, *]
        rw [Finset.coe_insert, Set.biUnion_insert, ← Set.union_self (f a : Set R),
          subset_union_prime' hp', ← or_assoc, or_self_iff] at h
        rwa [Finset.exists_mem_insert]
    · by_cases hbs : b ∈ s
      · obtain ⟨t, hbt, rfl⟩ : ∃ t, b ∉ t ∧ insert b t = s :=
          ⟨s.erase b, Finset.not_mem_erase b s, Finset.insert_erase hbs⟩
        have hp' : ∀ j ∈ t, IsPrime (f j) := by
          intro j hj
          refine' hp j (Finset.mem_insert_of_mem hj) _ _ <;> rintro rfl <;>
            solve_by_elim only [Finset.mem_insert_of_mem, *]
        rw [Finset.coe_insert, Set.biUnion_insert, ← Set.union_self (f b : Set R),
          subset_union_prime' hp', ← or_assoc, or_self_iff] at h
        rwa [Finset.exists_mem_insert]
      rcases s.eq_empty_or_nonempty with hse | hsne
      · subst hse
        rw [Finset.coe_empty, Set.biUnion_empty, Set.subset_empty_iff] at h
        have : (I : Set R) ≠ ∅ := Set.Nonempty.ne_empty (Set.nonempty_of_mem I.zero_mem)
        exact absurd h this
      · cases' hsne.bex with i his
        obtain ⟨t, _, rfl⟩ : ∃ t, i ∉ t ∧ insert i t = s :=
          ⟨s.erase i, Finset.not_mem_erase i s, Finset.insert_erase his⟩
        have hp' : ∀ j ∈ t, IsPrime (f j) := by
          intro j hj
          refine' hp j (Finset.mem_insert_of_mem hj) _ _ <;> rintro rfl <;>
            solve_by_elim only [Finset.mem_insert_of_mem, *]
        rw [Finset.coe_insert, Set.biUnion_insert, ← Set.union_self (f i : Set R),
          subset_union_prime' hp', ← or_assoc, or_self_iff] at h
        rwa [Finset.exists_mem_insert]