-- Mathlib/Topology/Compactness/Lindelof.lean
theorem IsLindelof.elim_countable_subcover {ι : Type v} (hs : IsLindelof s) (U : ι → Set X)
    (hUo : ∀ i, IsOpen (U i)) (hsU : s ⊆ ⋃ i, U i) :
    ∃ r : Set ι, r.Countable ∧ (s ⊆ ⋃ i ∈ r, U i)

Proof state:
X : Type u
inst✝ : TopologicalSpace X
s : Set X
ι : Type v
hs : IsLindelof s
U : ι → Set X
hUo : ∀ (i : ι), IsOpen (U i)
hsU : s ⊆ ⋃ i, U i
⊢ ∀ ⦃s t : Set X⦄, s ⊆ t → (∃ r, r.Countable ∧ t ⊆ ⋃ i ∈ r, U i) → ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i

Tactic:
intro _ _ hst ⟨r, ⟨hrcountable, hsub⟩⟩

Result:
X : Type u
inst✝ : TopologicalSpace X
s : Set X
ι : Type v
hs : IsLindelof s
U : ι → Set X
hUo : ∀ (i : ι), IsOpen (U i)
hsU : s ⊆ ⋃ i, U i
s✝ t✝ : Set X
hst : s✝ ⊆ t✝
r : Set ι
hrcountable : r.Countable
hsub : t✝ ⊆ ⋃ i ∈ r, U i
⊢ ∃ r, r.Countable ∧ s✝ ⊆ ⋃ i ∈ r, U i

Proof state:
X : Type u
inst✝ : TopologicalSpace X
s : Set X
ι : Type v
hs : IsLindelof s
U : ι → Set X
hUo : ∀ (i : ι), IsOpen (U i)
hsU : s ⊆ ⋃ i, U i
hmono : ∀ ⦃s t : Set X⦄, s ⊆ t → (∃ r, r.Countable ∧ t ⊆ ⋃ i ∈ r, U i) → ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i
hcountable_union :
  ∀ (S : Set (Set X)),
    S.Countable → (∀ s ∈ S, ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i) → ∃ r, r.Countable ∧ ⋃₀ S ⊆ ⋃ i ∈ r, U i
x : X
hx : x ∈ s
i : ι
hi : x ∈ U i
⊢ U i ⊆ U i

Tactic:
exact Subset.refl _

Result:
no goals

Proof state:
X : Type u
inst✝ : TopologicalSpace X
s : Set X
ι : Type v
hs : IsLindelof s
U : ι → Set X
hUo : ∀ (i : ι), IsOpen (U i)
hsU : s ⊆ ⋃ i, U i
hmono : ∀ ⦃s t : Set X⦄, s ⊆ t → (∃ r, r.Countable ∧ t ⊆ ⋃ i ∈ r, U i) → ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i
hcountable_union :
  ∀ (S : Set (Set X)),
    S.Countable → (∀ s ∈ S, ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i) → ∃ r, r.Countable ∧ ⋃₀ S ⊆ ⋃ i ∈ r, U i
x : X
hx : x ∈ s
⊢ ∃ t ∈ 𝓝[s] x, ∃ r, r.Countable ∧ t ⊆ ⋃ i ∈ r, U i

Tactic:
let ⟨i, hi⟩ := mem_iUnion.1 (hsU hx)

Result:
X : Type u
inst✝ : TopologicalSpace X
s : Set X
ι : Type v
hs : IsLindelof s
U : ι → Set X
hUo : ∀ (i : ι), IsOpen (U i)
hsU : s ⊆ ⋃ i, U i
hmono : ∀ ⦃s t : Set X⦄, s ⊆ t → (∃ r, r.Countable ∧ t ⊆ ⋃ i ∈ r, U i) → ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i
hcountable_union :
  ∀ (S : Set (Set X)),
    S.Countable → (∀ s ∈ S, ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i) → ∃ r, r.Countable ∧ ⋃₀ S ⊆ ⋃ i ∈ r, U i
x : X
hx : x ∈ s
i : ι
hi : x ∈ U i
⊢ ∃ t ∈ 𝓝[s] x, ∃ r, r.Countable ∧ t ⊆ ⋃ i ∈ r, U i

Proof state:
X : Type u
inst✝ : TopologicalSpace X
s : Set X
ι : Type v
hs : IsLindelof s
U : ι → Set X
hUo : ∀ (i : ι), IsOpen (U i)
hsU : s ⊆ ⋃ i, U i
hmono : ∀ ⦃s t : Set X⦄, s ⊆ t → (∃ r, r.Countable ∧ t ⊆ ⋃ i ∈ r, U i) → ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i
hcountable_union :
  ∀ (S : Set (Set X)),
    S.Countable → (∀ s ∈ S, ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i) → ∃ r, r.Countable ∧ ⋃₀ S ⊆ ⋃ i ∈ r, U i
x : X
hx : x ∈ s
i : ι
hi : x ∈ U i
⊢ U i ⊆ ⋃ i_1 ∈ {i}, U i_1

Tactic:
simp only [mem_singleton_iff, iUnion_iUnion_eq_left]

Result:
X : Type u
inst✝ : TopologicalSpace X
s : Set X
ι : Type v
hs : IsLindelof s
U : ι → Set X
hUo : ∀ (i : ι), IsOpen (U i)
hsU : s ⊆ ⋃ i, U i
hmono : ∀ ⦃s t : Set X⦄, s ⊆ t → (∃ r, r.Countable ∧ t ⊆ ⋃ i ∈ r, U i) → ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i
hcountable_union :
  ∀ (S : Set (Set X)),
    S.Countable → (∀ s ∈ S, ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i) → ∃ r, r.Countable ∧ ⋃₀ S ⊆ ⋃ i ∈ r, U i
x : X
hx : x ∈ s
i : ι
hi : x ∈ U i
⊢ U i ⊆ U i

Proof state:
X : Type u
inst✝ : TopologicalSpace X
s : Set X
ι : Type v
hs : IsLindelof s
U : ι → Set X
hUo : ∀ (i : ι), IsOpen (U i)
hsU : s ⊆ ⋃ i, U i
hmono : ∀ ⦃s t : Set X⦄, s ⊆ t → (∃ r, r.Countable ∧ t ⊆ ⋃ i ∈ r, U i) → ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i
S : Set (Set X)
hS : S.Countable
r : Set X → Set ι
hr : ∀ s ∈ S, (r s).Countable ∧ s ⊆ ⋃ i ∈ r s, U i
⊢ ⋃₀ S ⊆ ⋃ i ∈ ⋃ s ∈ S, r s, U i

Tactic:
refine sUnion_subset ?h.right.h

Result:
case h.right.h
X : Type u
inst✝ : TopologicalSpace X
s : Set X
ι : Type v
hs : IsLindelof s
U : ι → Set X
hUo : ∀ (i : ι), IsOpen (U i)
hsU : s ⊆ ⋃ i, U i
hmono : ∀ ⦃s t : Set X⦄, s ⊆ t → (∃ r, r.Countable ∧ t ⊆ ⋃ i ∈ r, U i) → ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i
S : Set (Set X)
hS : S.Countable
r : Set X → Set ι
hr : ∀ s ∈ S, (r s).Countable ∧ s ⊆ ⋃ i ∈ r s, U i
⊢ ∀ t' ∈ S, t' ⊆ ⋃ i ∈ ⋃ s ∈ S, r s, U i

Proof state:
X : Type u
inst✝ : TopologicalSpace X
s : Set X
ι : Type v
hs : IsLindelof s
U : ι → Set X
hUo : ∀ (i : ι), IsOpen (U i)
hsU : s ⊆ ⋃ i, U i
hmono : ∀ ⦃s t : Set X⦄, s ⊆ t → (∃ r, r.Countable ∧ t ⊆ ⋃ i ∈ r, U i) → ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i
hcountable_union :
  ∀ (S : Set (Set X)),
    S.Countable → (∀ s ∈ S, ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i) → ∃ r, r.Countable ∧ ⋃₀ S ⊆ ⋃ i ∈ r, U i
⊢ ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i

Tactic:
have h_nhds : ∀ x ∈ s, ∃ t ∈ 𝓝[s] x, ∃ r : Set ι, r.Countable ∧ (t ⊆ ⋃ i ∈ r, U i) := by
  intro x hx
  let ⟨i, hi⟩ := mem_iUnion.1 (hsU hx)
  refine ⟨U i, mem_nhdsWithin_of_mem_nhds ((hUo i).mem_nhds hi), {i}, by simp, ?_⟩
  simp only [mem_singleton_iff, iUnion_iUnion_eq_left]
  exact Subset.refl _

Result:
X : Type u
inst✝ : TopologicalSpace X
s : Set X
ι : Type v
hs : IsLindelof s
U : ι → Set X
hUo : ∀ (i : ι), IsOpen (U i)
hsU : s ⊆ ⋃ i, U i
hmono : ∀ ⦃s t : Set X⦄, s ⊆ t → (∃ r, r.Countable ∧ t ⊆ ⋃ i ∈ r, U i) → ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i
hcountable_union :
  ∀ (S : Set (Set X)),
    S.Countable → (∀ s ∈ S, ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i) → ∃ r, r.Countable ∧ ⋃₀ S ⊆ ⋃ i ∈ r, U i
h_nhds : ∀ x ∈ s, ∃ t ∈ 𝓝[s] x, ∃ r, r.Countable ∧ t ⊆ ⋃ i ∈ r, U i
⊢ ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i

Proof state:
case h.right.h
X : Type u
inst✝ : TopologicalSpace X
s : Set X
ι : Type v
hs : IsLindelof s
U : ι → Set X
hUo : ∀ (i : ι), IsOpen (U i)
hsU : s ⊆ ⋃ i, U i
hmono : ∀ ⦃s t : Set X⦄, s ⊆ t → (∃ r, r.Countable ∧ t ⊆ ⋃ i ∈ r, U i) → ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i
S : Set (Set X)
hS : S.Countable
r : Set X → Set ι
hr : ∀ s ∈ S, (r s).Countable ∧ s ⊆ ⋃ i ∈ r s, U i
⊢ ∀ t' ∈ S, t' ⊆ ⋃ i ∈ ⋃ s ∈ S, r s, U i

Tactic:
simp only [mem_iUnion, exists_prop, iUnion_exists, biUnion_and']

Result:
case h.right.h
X : Type u
inst✝ : TopologicalSpace X
s : Set X
ι : Type v
hs : IsLindelof s
U : ι → Set X
hUo : ∀ (i : ι), IsOpen (U i)
hsU : s ⊆ ⋃ i, U i
hmono : ∀ ⦃s t : Set X⦄, s ⊆ t → (∃ r, r.Countable ∧ t ⊆ ⋃ i ∈ r, U i) → ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i
S : Set (Set X)
hS : S.Countable
r : Set X → Set ι
hr : ∀ s ∈ S, (r s).Countable ∧ s ⊆ ⋃ i ∈ r s, U i
⊢ ∀ t' ∈ S, t' ⊆ ⋃ y ∈ S, ⋃ x ∈ r y, U x

Proof state:
X : Type u
inst✝ : TopologicalSpace X
s : Set X
ι : Type v
hs : IsLindelof s
U : ι → Set X
hUo : ∀ (i : ι), IsOpen (U i)
hsU : s ⊆ ⋃ i, U i
hmono : ∀ ⦃s t : Set X⦄, s ⊆ t → (∃ r, r.Countable ∧ t ⊆ ⋃ i ∈ r, U i) → ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i
hcountable_union :
  ∀ (S : Set (Set X)),
    S.Countable → (∀ s ∈ S, ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i) → ∃ r, r.Countable ∧ ⋃₀ S ⊆ ⋃ i ∈ r, U i
h_nhds : ∀ x ∈ s, ∃ t ∈ 𝓝[s] x, ∃ r, r.Countable ∧ t ⊆ ⋃ i ∈ r, U i
⊢ ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i

Tactic:
exact hs.induction_on hmono hcountable_union h_nhds

Result:
no goals

Proof state:
X : Type u
inst✝ : TopologicalSpace X
s : Set X
ι : Type v
hs : IsLindelof s
U : ι → Set X
hUo : ∀ (i : ι), IsOpen (U i)
hsU : s ⊆ ⋃ i, U i
hmono : ∀ ⦃s t : Set X⦄, s ⊆ t → (∃ r, r.Countable ∧ t ⊆ ⋃ i ∈ r, U i) → ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i
⊢ ∀ (S : Set (Set X)),
    S.Countable → (∀ s ∈ S, ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i) → ∃ r, r.Countable ∧ ⋃₀ S ⊆ ⋃ i ∈ r, U i

Tactic:
intro S hS hsr

Result:
X : Type u
inst✝ : TopologicalSpace X
s : Set X
ι : Type v
hs : IsLindelof s
U : ι → Set X
hUo : ∀ (i : ι), IsOpen (U i)
hsU : s ⊆ ⋃ i, U i
hmono : ∀ ⦃s t : Set X⦄, s ⊆ t → (∃ r, r.Countable ∧ t ⊆ ⋃ i ∈ r, U i) → ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i
S : Set (Set X)
hS : S.Countable
hsr : ∀ s ∈ S, ∃ r, r.Countable ∧ s ⊆ ⋃ i ∈ r, U i
⊢ ∃ r, r.Countable ∧ ⋃₀ S ⊆ ⋃ i ∈ r, U i