-- Mathlib/Order/LiminfLimsup.lean
theorem HasBasis.liminf_eq_ciSup_ciInf {v : Filter ι}
    {p : ι' → Prop} {s : ι' → Set ι} [Countable (Subtype p)] [Nonempty (Subtype p)]
    (hv : v.HasBasis p s) {f : ι → α} (hs : ∀ (j : Subtype p), (s j).Nonempty)
    (H : ∃ (j : Subtype p), BddBelow (range (fun (i : s j) ↦ f i))) :
    liminf f v = ⨆ (j : Subtype p), ⨅ (i : s (liminf_reparam f s p j)), f i

Proof state:
case neg
α : Type u_1
ι : Type u_4
ι' : Type u_5
inst✝² : ConditionallyCompleteLinearOrder α
v : Filter ι
p : ι' → Prop
s : ι' → Set ι
inst✝¹ : Countable (Subtype p)
inst✝ : Nonempty (Subtype p)
hv : v.HasBasis p s
f : ι → α
hs : ∀ (j : Subtype p), (s ↑j).Nonempty
j0 : Subtype p
hj0 : BddBelow (range fun i => f ↑i)
m : Set (Subtype p) := {j | BddBelow (range fun i => f ↑i)}
this : ∀ (j : Subtype p), Nonempty ↑(s ↑j)
A : ⋃ j, ⋂ i, Iic (f ↑i) = ⋃ j, ⋂ i, Iic (f ↑i)
j : Subtype p
Hj : j ∉ m
⊢ liminf_reparam f s p j ∈ m

Tactic:
simp only [m, liminf_reparam, if_neg Hj]

Result:
case neg
α : Type u_1
ι : Type u_4
ι' : Type u_5
inst✝² : ConditionallyCompleteLinearOrder α
v : Filter ι
p : ι' → Prop
s : ι' → Set ι
inst✝¹ : Countable (Subtype p)
inst✝ : Nonempty (Subtype p)
hv : v.HasBasis p s
f : ι → α
hs : ∀ (j : Subtype p), (s ↑j).Nonempty
j0 : Subtype p
hj0 : BddBelow (range fun i => f ↑i)
m : Set (Subtype p) := {j | BddBelow (range fun i => f ↑i)}
this : ∀ (j : Subtype p), Nonempty ↑(s ↑j)
A : ⋃ j, ⋂ i, Iic (f ↑i) = ⋃ j, ⋂ i, Iic (f ↑i)
j : Subtype p
Hj : j ∉ m
⊢ ⋯.choose (Nat.find ⋯) ∈ {j | BddBelow (range fun i => f ↑i)}

Proof state:
α : Type u_1
ι : Type u_4
ι' : Type u_5
inst✝² : ConditionallyCompleteLinearOrder α
v : Filter ι
p : ι' → Prop
s : ι' → Set ι
inst✝¹ : Countable (Subtype p)
inst✝ : Nonempty (Subtype p)
hv : v.HasBasis p s
f : ι → α
hs : ∀ (j : Subtype p), (s ↑j).Nonempty
j0 : Subtype p
hj0 : BddBelow (range fun i => f ↑i)
m : Set (Subtype p) := {j | BddBelow (range fun i => f ↑i)}
this : ∀ (j : Subtype p), Nonempty ↑(s ↑j)
j : Subtype p
⊢ ⋂ i, Iic (f ↑i) ⊆ ⋃ j, ⋂ i, Iic (f ↑i)

Tactic:
by_cases hj : j ∈ m

Result:
case pos
α : Type u_1
ι : Type u_4
ι' : Type u_5
inst✝² : ConditionallyCompleteLinearOrder α
v : Filter ι
p : ι' → Prop
s : ι' → Set ι
inst✝¹ : Countable (Subtype p)
inst✝ : Nonempty (Subtype p)
hv : v.HasBasis p s
f : ι → α
hs : ∀ (j : Subtype p), (s ↑j).Nonempty
j0 : Subtype p
hj0 : BddBelow (range fun i => f ↑i)
m : Set (Subtype p) := {j | BddBelow (range fun i => f ↑i)}
this : ∀ (j : Subtype p), Nonempty ↑(s ↑j)
j : Subtype p
hj : j ∈ m
⊢ ⋂ i, Iic (f ↑i) ⊆ ⋃ j, ⋂ i, Iic (f ↑i)

case neg
α : Type u_1
ι : Type u_4
ι' : Type u_5
inst✝² : ConditionallyCompleteLinearOrder α
v : Filter ι
p : ι' → Prop
s : ι' → Set ι
inst✝¹ : Countable (Subtype p)
inst✝ : Nonempty (Subtype p)
hv : v.HasBasis p s
f : ι → α
hs : ∀ (j : Subtype p), (s ↑j).Nonempty
j0 : Subtype p
hj0 : BddBelow (range fun i => f ↑i)
m : Set (Subtype p) := {j | BddBelow (range fun i => f ↑i)}
this : ∀ (j : Subtype p), Nonempty ↑(s ↑j)
j : Subtype p
hj : j ∉ m
⊢ ⋂ i, Iic (f ↑i) ⊆ ⋃ j, ⋂ i, Iic (f ↑i)

Proof state:
case neg
α : Type u_1
ι : Type u_4
ι' : Type u_5
inst✝² : ConditionallyCompleteLinearOrder α
v : Filter ι
p : ι' → Prop
s : ι' → Set ι
inst✝¹ : Countable (Subtype p)
inst✝ : Nonempty (Subtype p)
hv : v.HasBasis p s
f : ι → α
hs : ∀ (j : Subtype p), (s ↑j).Nonempty
j0 : Subtype p
hj0 : BddBelow (range fun i => f ↑i)
m : Set (Subtype p) := {j | BddBelow (range fun i => f ↑i)}
this : ∀ (j : Subtype p), Nonempty ↑(s ↑j)
A : ⋃ j, ⋂ i, Iic (f ↑i) = ⋃ j, ⋂ i, Iic (f ↑i)
j : Subtype p
Hj : j ∉ m
Z : ∃ n, ⋯.choose n ∈ m ∨ ∀ (j : Subtype p), j ∉ m
⊢ ⋯.choose (Nat.find ⋯) ∈ {j | BddBelow (range fun i => f ↑i)}

Tactic:
rcases Nat.find_spec Z with hZ|hZ

Result:
case neg.inl
α : Type u_1
ι : Type u_4
ι' : Type u_5
inst✝² : ConditionallyCompleteLinearOrder α
v : Filter ι
p : ι' → Prop
s : ι' → Set ι
inst✝¹ : Countable (Subtype p)
inst✝ : Nonempty (Subtype p)
hv : v.HasBasis p s
f : ι → α
hs : ∀ (j : Subtype p), (s ↑j).Nonempty
j0 : Subtype p
hj0 : BddBelow (range fun i => f ↑i)
m : Set (Subtype p) := {j | BddBelow (range fun i => f ↑i)}
this : ∀ (j : Subtype p), Nonempty ↑(s ↑j)
A : ⋃ j, ⋂ i, Iic (f ↑i) = ⋃ j, ⋂ i, Iic (f ↑i)
j : Subtype p
Hj : j ∉ m
Z : ∃ n, ⋯.choose n ∈ m ∨ ∀ (j : Subtype p), j ∉ m
hZ : ⋯.choose (Nat.find Z) ∈ m
⊢ ⋯.choose (Nat.find ⋯) ∈ {j | BddBelow (range fun i => f ↑i)}

case neg.inr
α : Type u_1
ι : Type u_4
ι' : Type u_5
inst✝² : ConditionallyCompleteLinearOrder α
v : Filter ι
p : ι' → Prop
s : ι' → Set ι
inst✝¹ : Countable (Subtype p)
inst✝ : Nonempty (Subtype p)
hv : v.HasBasis p s
f : ι → α
hs : ∀ (j : Subtype p), (s ↑j).Nonempty
j0 : Subtype p
hj0 : BddBelow (range fun i => f ↑i)
m : Set (Subtype p) := {j | BddBelow (range fun i => f ↑i)}
this : ∀ (j : Subtype p), Nonempty ↑(s ↑j)
A : ⋃ j, ⋂ i, Iic (f ↑i) = ⋃ j, ⋂ i, Iic (f ↑i)
j : Subtype p
Hj : j ∉ m
Z : ∃ n, ⋯.choose n ∈ m ∨ ∀ (j : Subtype p), j ∉ m
hZ : ∀ (j : Subtype p), j ∉ m
⊢ ⋯.choose (Nat.find ⋯) ∈ {j | BddBelow (range fun i => f ↑i)}