Module: Mathlib.NumberTheory.WellApproximable

Goal state before:
pp: T : ℝ
hT : Fact (0 < T)
δ : ℕ → ℝ
hδ : Tendsto δ atTop (𝓝 0)
this : SemilatticeSup Nat.Primes := Nat.Subtype.semilatticeSup fun p ↦ Nat.Prime p
μ : Measure 𝕊 := volume
u : Nat.Primes → 𝕊 := fun p ↦ ↑(↑1 / ↑↑p * T)
hu₀ : ∀ (p : Nat.Primes), addOrderOf (u p) = ↑p
hu : Tendsto (addOrderOf ∘ u) atTop atTop
E : Set 𝕊 := addWellApproximable 𝕊 δ
X : ℕ → Set 𝕊 := fun n ↦ approxAddOrderOf 𝕊 n (δ n)
A : ℕ → Set 𝕊 := fun p ↦ blimsup X atTop fun n ↦ 0 < n ∧ ¬p ∣ n
B : ℕ → Set 𝕊 := fun p ↦ blimsup X atTop fun n ↦ 0 < n ∧ p ∣ n ∧ ¬p * p ∣ n
C : ℕ → Set 𝕊 := fun p ↦ blimsup X atTop fun n ↦ 0 < n ∧ p ^ 2 ∣ n
hA₀ : ∀ (p : ℕ), MeasurableSet (A p)
hB₀ : ∀ (p : ℕ), MeasurableSet (B p)
hE₀ : NullMeasurableSet E μ
hE₁ : ∀ (p : ℕ), E = A p ∪ B p ∪ C p
hE₂ : ∀ (p : Nat.Primes), A ↑p =ᵐ[μ] ∅ ∧ B ↑p =ᵐ[μ] ∅ → E =ᵐ[μ] C ↑p
hA : ∀ (p : Nat.Primes), A ↑p =ᵐ[μ] ∅ ∨ A ↑p =ᵐ[μ] univ
hB : ∀ (p : Nat.Primes), B ↑p =ᵐ[μ] ∅ ∨ B ↑p =ᵐ[μ] univ
hC : ∀ (p : Nat.Primes), u p +ᵥ C ↑p = C ↑p
⊢ E =ᵐ[volume] ∅ ∨ E =ᵐ[volume] univ

Tactic:
by_cases! +distrib h : ∀ p : Nat.Primes, A p =ᵐ[μ] (∅ : Set 𝕊) ∧ B p =ᵐ[μ] (∅ : Set 𝕊)

Goal state after:
case pos
T : ℝ
hT : Fact (0 < T)
δ : ℕ → ℝ
hδ : Tendsto δ atTop (𝓝 0)
this : SemilatticeSup Nat.Primes := Nat.Subtype.semilatticeSup fun p ↦ Nat.Prime p
μ : Measure 𝕊 := volume
u : Nat.Primes → 𝕊 := fun p ↦ ↑(↑1 / ↑↑p * T)
hu₀ : ∀ (p : Nat.Primes), addOrderOf (u p) = ↑p
hu : Tendsto (addOrderOf ∘ u) atTop atTop
E : Set 𝕊 := addWellApproximable 𝕊 δ
X : ℕ → Set 𝕊 := fun n ↦ approxAddOrderOf 𝕊 n (δ n)
A : ℕ → Set 𝕊 := fun p ↦ blimsup X atTop fun n ↦ 0 < n ∧ ¬p ∣ n
B : ℕ → Set 𝕊 := fun p ↦ blimsup X atTop fun n ↦ 0 < n ∧ p ∣ n ∧ ¬p * p ∣ n
C : ℕ → Set 𝕊 := fun p ↦ blimsup X atTop fun n ↦ 0 < n ∧ p ^ 2 ∣ n
hA₀ : ∀ (p : ℕ), MeasurableSet (A p)
hB₀ : ∀ (p : ℕ), MeasurableSet (B p)
hE₀ : NullMeasurableSet E μ
hE₁ : ∀ (p : ℕ), E = A p ∪ B p ∪ C p
hE₂ : ∀ (p : Nat.Primes), A ↑p =ᵐ[μ] ∅ ∧ B ↑p =ᵐ[μ] ∅ → E =ᵐ[μ] C ↑p
hA : ∀ (p : Nat.Primes), A ↑p =ᵐ[μ] ∅ ∨ A ↑p =ᵐ[μ] univ
hB : ∀ (p : Nat.Primes), B ↑p =ᵐ[μ] ∅ ∨ B ↑p =ᵐ[μ] univ
hC : ∀ (p : Nat.Primes), u p +ᵥ C ↑p = C ↑p
h : ∀ (p : Nat.Primes), A ↑p =ᵐ[μ] ∅ ∧ B ↑p =ᵐ[μ] ∅
⊢ E =ᵐ[volume] ∅ ∨ E =ᵐ[volume] univ

case neg
T : ℝ
hT : Fact (0 < T)
δ : ℕ → ℝ
hδ : Tendsto δ atTop (𝓝 0)
this : SemilatticeSup Nat.Primes := Nat.Subtype.semilatticeSup fun p ↦ Nat.Prime p
μ : Measure 𝕊 := volume
u : Nat.Primes → 𝕊 := fun p ↦ ↑(↑1 / ↑↑p * T)
hu₀ : ∀ (p : Nat.Primes), addOrderOf (u p) = ↑p
hu : Tendsto (addOrderOf ∘ u) atTop atTop
E : Set 𝕊 := addWellApproximable 𝕊 δ
X : ℕ → Set 𝕊 := fun n ↦ approxAddOrderOf 𝕊 n (δ n)
A : ℕ → Set 𝕊 := fun p ↦ blimsup X atTop fun n ↦ 0 < n ∧ ¬p ∣ n
B : ℕ → Set 𝕊 := fun p ↦ blimsup X atTop fun n ↦ 0 < n ∧ p ∣ n ∧ ¬p * p ∣ n
C : ℕ → Set 𝕊 := fun p ↦ blimsup X atTop fun n ↦ 0 < n ∧ p ^ 2 ∣ n
hA₀ : ∀ (p : ℕ), MeasurableSet (A p)
hB₀ : ∀ (p : ℕ), MeasurableSet (B p)
hE₀ : NullMeasurableSet E μ
hE₁ : ∀ (p : ℕ), E = A p ∪ B p ∪ C p
hE₂ : ∀ (p : Nat.Primes), A ↑p =ᵐ[μ] ∅ ∧ B ↑p =ᵐ[μ] ∅ → E =ᵐ[μ] C ↑p
hA : ∀ (p : Nat.Primes), A ↑p =ᵐ[μ] ∅ ∨ A ↑p =ᵐ[μ] univ
hB : ∀ (p : Nat.Primes), B ↑p =ᵐ[μ] ∅ ∨ B ↑p =ᵐ[μ] univ
hC : ∀ (p : Nat.Primes), u p +ᵥ C ↑p = C ↑p
h : ∃ p, ¬A ↑p =ᵐ[μ] ∅ ∨ ¬B ↑p =ᵐ[μ] ∅
⊢ E =ᵐ[volume] ∅ ∨ E =ᵐ[volume] univ