-- Mathlib/MeasureTheory/Constructions/BorelSpace/Order.lean
theorem ext_of_Ico' {α : Type*} [TopologicalSpace α] {m : MeasurableSpace α}
    [SecondCountableTopology α] [LinearOrder α] [OrderTopology α] [BorelSpace α] [NoMaxOrder α]
    (μ ν : Measure α) (hμ : ∀ ⦃a b⦄, a < b → μ (Ico a b) ≠ ∞)
    (h : ∀ ⦃a b⦄, a < b → μ (Ico a b) = ν (Ico a b)) : μ = ν

Proof state:
case intro.intro.intro.intro
α : Type u_5
inst✝⁵ : TopologicalSpace α
m : MeasurableSpace α
inst✝⁴ : SecondCountableTopology α
inst✝³ : LinearOrder α
inst✝² : OrderTopology α
inst✝¹ : BorelSpace α
inst✝ : NoMaxOrder α
μ ν : Measure α
hμ : ∀ ⦃a b : α⦄, a < b → μ (Ico a b) ≠ ⊤
h : ∀ ⦃a b : α⦄, a < b → μ (Ico a b) = ν (Ico a b)
s : Set α
hsc : s.Countable
hsd : Dense s
hsb : ∀ (x : α), IsBot x → x ∈ s
right✝ : ∀ (x : α), IsTop x → x ∈ s
this : {x | ∃ i, ∃ (_ : i ∈ s), ∃ i_2, ∃ (_ : i_2 ∈ s) (_ : i < i_2), x = Ico i i_2}.Countable
⊢ μ = ν

Tactic:
refine
  Measure.ext_of_generateFrom_of_cover_subset
    (BorelSpace.measurable_eq.trans (borel_eq_generateFrom_Ico α)) (isPiSystem_Ico id id) ?_ this
    ?_ ?_ ?_

Result:
case intro.intro.intro.intro.refine_1
α : Type u_5
inst✝⁵ : TopologicalSpace α
m : MeasurableSpace α
inst✝⁴ : SecondCountableTopology α
inst✝³ : LinearOrder α
inst✝² : OrderTopology α
inst✝¹ : BorelSpace α
inst✝ : NoMaxOrder α
μ ν : Measure α
hμ : ∀ ⦃a b : α⦄, a < b → μ (Ico a b) ≠ ⊤
h : ∀ ⦃a b : α⦄, a < b → μ (Ico a b) = ν (Ico a b)
s : Set α
hsc : s.Countable
hsd : Dense s
hsb : ∀ (x : α), IsBot x → x ∈ s
right✝ : ∀ (x : α), IsTop x → x ∈ s
this : {x | ∃ i, ∃ (_ : i ∈ s), ∃ i_2, ∃ (_ : i_2 ∈ s) (_ : i < i_2), x = Ico i i_2}.Countable
⊢ {x | ∃ i, ∃ (_ : i ∈ s), ∃ i_2, ∃ (_ : i_2 ∈ s) (_ : i < i_2), x = Ico i i_2} ⊆ {S | ∃ l u, l < u ∧ Ico l u = S}

case intro.intro.intro.intro.refine_2
α : Type u_5
inst✝⁵ : TopologicalSpace α
m : MeasurableSpace α
inst✝⁴ : SecondCountableTopology α
inst✝³ : LinearOrder α
inst✝² : OrderTopology α
inst✝¹ : BorelSpace α
inst✝ : NoMaxOrder α
μ ν : Measure α
hμ : ∀ ⦃a b : α⦄, a < b → μ (Ico a b) ≠ ⊤
h : ∀ ⦃a b : α⦄, a < b → μ (Ico a b) = ν (Ico a b)
s : Set α
hsc : s.Countable
hsd : Dense s
hsb : ∀ (x : α), IsBot x → x ∈ s
right✝ : ∀ (x : α), IsTop x → x ∈ s
this : {x | ∃ i, ∃ (_ : i ∈ s), ∃ i_2, ∃ (_ : i_2 ∈ s) (_ : i < i_2), x = Ico i i_2}.Countable
⊢ ⋃₀ {x | ∃ i, ∃ (_ : i ∈ s), ∃ i_2, ∃ (_ : i_2 ∈ s) (_ : i < i_2), x = Ico i i_2} = univ

case intro.intro.intro.intro.refine_3
α : Type u_5
inst✝⁵ : TopologicalSpace α
m : MeasurableSpace α
inst✝⁴ : SecondCountableTopology α
inst✝³ : LinearOrder α
inst✝² : OrderTopology α
inst✝¹ : BorelSpace α
inst✝ : NoMaxOrder α
μ ν : Measure α
hμ : ∀ ⦃a b : α⦄, a < b → μ (Ico a b) ≠ ⊤
h : ∀ ⦃a b : α⦄, a < b → μ (Ico a b) = ν (Ico a b)
s : Set α
hsc : s.Countable
hsd : Dense s
hsb : ∀ (x : α), IsBot x → x ∈ s
right✝ : ∀ (x : α), IsTop x → x ∈ s
this : {x | ∃ i, ∃ (_ : i ∈ s), ∃ i_2, ∃ (_ : i_2 ∈ s) (_ : i < i_2), x = Ico i i_2}.Countable
⊢ ∀ s_1 ∈ {x | ∃ i, ∃ (_ : i ∈ s), ∃ i_2, ∃ (_ : i_2 ∈ s) (_ : i < i_2), x = Ico i i_2}, μ s_1 ≠ ⊤

case intro.intro.intro.intro.refine_4
α : Type u_5
inst✝⁵ : TopologicalSpace α
m : MeasurableSpace α
inst✝⁴ : SecondCountableTopology α
inst✝³ : LinearOrder α
inst✝² : OrderTopology α
inst✝¹ : BorelSpace α
inst✝ : NoMaxOrder α
μ ν : Measure α
hμ : ∀ ⦃a b : α⦄, a < b → μ (Ico a b) ≠ ⊤
h : ∀ ⦃a b : α⦄, a < b → μ (Ico a b) = ν (Ico a b)
s : Set α
hsc : s.Countable
hsd : Dense s
hsb : ∀ (x : α), IsBot x → x ∈ s
right✝ : ∀ (x : α), IsTop x → x ∈ s
this : {x | ∃ i, ∃ (_ : i ∈ s), ∃ i_2, ∃ (_ : i_2 ∈ s) (_ : i < i_2), x = Ico i i_2}.Countable
⊢ ∀ s ∈ {S | ∃ l u, l < u ∧ Ico l u = S}, μ s = ν s

Proof state:
case intro.intro.intro.intro.refine_2.intro.intro.intro.intro
α : Type u_5
inst✝⁵ : TopologicalSpace α
m : MeasurableSpace α
inst✝⁴ : SecondCountableTopology α
inst✝³ : LinearOrder α
inst✝² : OrderTopology α
inst✝¹ : BorelSpace α
inst✝ : NoMaxOrder α
μ ν : Measure α
hμ : ∀ ⦃a b : α⦄, a < b → μ (Ico a b) ≠ ⊤
h : ∀ ⦃a b : α⦄, a < b → μ (Ico a b) = ν (Ico a b)
s : Set α
hsc : s.Countable
hsd : Dense s
hsb : ∀ (x : α), IsBot x → x ∈ s
right✝ : ∀ (x : α), IsTop x → x ∈ s
this : {x | ∃ i, ∃ (_ : i ∈ s), ∃ i_2, ∃ (_ : i_2 ∈ s) (_ : i < i_2), x = Ico i i_2}.Countable
x l : α
hls : l ∈ s
hlx : l ≤ x
u : α
hus : u ∈ s
hxu : x < u
⊢ ∃ b ∈ {x | ∃ i, ∃ (_ : i ∈ s), ∃ i_2, ∃ (_ : i_2 ∈ s) (_ : i < i_2), x = Ico i i_2}, x ∈ b

Tactic:
exact ⟨_, ⟨l, hls, u, hus, hlx.trans_lt hxu, rfl⟩, hlx, hxu⟩

Result:
no goals