-- Mathlib/GroupTheory/CosetCover.lean
theorem exists_finiteIndex_of_leftCoset_cover_aux [DecidableEq (Subgroup G)]
    (j : ι) (hj : j ∈ s) (hcovers' : ⋃ i ∈ s.filter (H · = H j), g i • (H i : Set G) ≠ Set.univ) :
    ∃ i ∈ s, H i ≠ H j ∧ (H i).FiniteIndex

Proof state:
case neg
G : Type u_1
inst✝¹ : Group G
inst✝ : DecidableEq (Subgroup G)
n : ℕ
ih :
  ∀ m < n,
    ∀ {ι : Type u_2} {H : ι → Subgroup G} {g : ι → G} {s : Finset ι},
      ⋃ i ∈ s, g i • ↑(H i) = Set.univ →
        ∀ j ∈ s,
          ⋃ i ∈ Finset.filter (fun x => H x = H j) s, g i • ↑(H i) ≠ Set.univ →
            m = (Finset.image H s).card → ∃ i ∈ s, H i ≠ H j ∧ (H i).FiniteIndex
ι : Type u_2
H : ι → Subgroup G
g : ι → G
s : Finset ι
j : ι
hj : j ∈ s
hcovers'✝ : ⋃ i ∈ Finset.filter (fun x => H x = H j) s, g i • ↑(H i) ≠ Set.univ
hn : n = (Finset.image H s).card
x : G
hx : ∀ (y : G), y • ↑(H j) ⊆ ⋃ i ∈ Finset.filter (fun x => H x ≠ H j) s, (y * x⁻¹ * g i) • ↑(H i)
κ : Type u_2 :=
  { x // x ∈ Finset.filter (fun x => H x ≠ H j) s } × Option { x // x ∈ Finset.filter (fun x => H x = H j) s }
f : κ → G :=
  fun x_1 =>
    match x_1 with
    | (k₁, some k₂) => g ↑k₂ * x⁻¹ * g ↑k₁
    | (k₁, none) => g ↑k₁
K : κ → Subgroup G := fun k => H ↑k.1
hK' : ∀ (k : κ), K k ∈ (Finset.image H s).erase (H j)
hK : ∀ (k : κ), K k ≠ H j
hcovers : ⋃ k ∈ Finset.univ, f k • ↑(K k) = Set.univ
k : κ
hcovers' : ¬⋃ i ∈ Finset.filter (fun x => K x = K k) Finset.univ, f i • ↑(K i) = Set.univ
hn' : (Finset.image K Finset.univ).card < n
k' : κ
hk' : k' ∈ Finset.univ ∧ K k' ≠ K k ∧ (K k').FiniteIndex
⊢ ∃ i ∈ s, H i ≠ H j ∧ (H i).FiniteIndex

Tactic:
exact ⟨k'.1.1, Finset.mem_of_mem_filter k'.1.1 k'.1.2, hK k', hk'.2.2⟩

Result:
no goals

Proof state:
case neg
G : Type u_1
inst✝¹ : Group G
inst✝ : DecidableEq (Subgroup G)
n : ℕ
ih :
  ∀ m < n,
    ∀ {ι : Type u_2} {H : ι → Subgroup G} {g : ι → G} {s : Finset ι},
      ⋃ i ∈ s, g i • ↑(H i) = Set.univ →
        ∀ j ∈ s,
          ⋃ i ∈ Finset.filter (fun x => H x = H j) s, g i • ↑(H i) ≠ Set.univ →
            m = (Finset.image H s).card → ∃ i ∈ s, H i ≠ H j ∧ (H i).FiniteIndex
ι : Type u_2
H : ι → Subgroup G
g : ι → G
s : Finset ι
j : ι
hj : j ∈ s
hcovers'✝ : ⋃ i ∈ Finset.filter (fun x => H x = H j) s, g i • ↑(H i) ≠ Set.univ
hn : n = (Finset.image H s).card
x : G
hx : ∀ (y : G), y • ↑(H j) ⊆ ⋃ i ∈ Finset.filter (fun x => H x ≠ H j) s, (y * x⁻¹ * g i) • ↑(H i)
κ : Type u_2 :=
  { x // x ∈ Finset.filter (fun x => H x ≠ H j) s } × Option { x // x ∈ Finset.filter (fun x => H x = H j) s }
f : κ → G :=
  fun x_1 =>
    match x_1 with
    | (k₁, some k₂) => g ↑k₂ * x⁻¹ * g ↑k₁
    | (k₁, none) => g ↑k₁
K : κ → Subgroup G := fun k => H ↑k.1
hK' : ∀ (k : κ), K k ∈ (Finset.image H s).erase (H j)
hK : ∀ (k : κ), K k ≠ H j
hcovers : ⋃ k ∈ Finset.univ, f k • ↑(K k) = Set.univ
k : κ
hcovers' : ¬⋃ i ∈ Finset.filter (fun x => K x = K k) Finset.univ, f i • ↑(K i) = Set.univ
⊢ ∃ i ∈ s, H i ≠ H j ∧ (H i).FiniteIndex

Tactic:
have hn' : (Finset.univ.image K).card < n := hn ▸ by
  refine ((Finset.card_le_card fun x => ?_).trans_lt <|
    Finset.card_erase_lt_of_mem (Finset.mem_image_of_mem H hj))
  rw [mem_image_univ_iff_mem_range, Set.mem_range]
  exact fun ⟨k, hk⟩ => hk ▸ hK' k

Result:
case neg
G : Type u_1
inst✝¹ : Group G
inst✝ : DecidableEq (Subgroup G)
n : ℕ
ih :
  ∀ m < n,
    ∀ {ι : Type u_2} {H : ι → Subgroup G} {g : ι → G} {s : Finset ι},
      ⋃ i ∈ s, g i • ↑(H i) = Set.univ →
        ∀ j ∈ s,
          ⋃ i ∈ Finset.filter (fun x => H x = H j) s, g i • ↑(H i) ≠ Set.univ →
            m = (Finset.image H s).card → ∃ i ∈ s, H i ≠ H j ∧ (H i).FiniteIndex
ι : Type u_2
H : ι → Subgroup G
g : ι → G
s : Finset ι
j : ι
hj : j ∈ s
hcovers'✝ : ⋃ i ∈ Finset.filter (fun x => H x = H j) s, g i • ↑(H i) ≠ Set.univ
hn : n = (Finset.image H s).card
x : G
hx : ∀ (y : G), y • ↑(H j) ⊆ ⋃ i ∈ Finset.filter (fun x => H x ≠ H j) s, (y * x⁻¹ * g i) • ↑(H i)
κ : Type u_2 :=
  { x // x ∈ Finset.filter (fun x => H x ≠ H j) s } × Option { x // x ∈ Finset.filter (fun x => H x = H j) s }
f : κ → G :=
  fun x_1 =>
    match x_1 with
    | (k₁, some k₂) => g ↑k₂ * x⁻¹ * g ↑k₁
    | (k₁, none) => g ↑k₁
K : κ → Subgroup G := fun k => H ↑k.1
hK' : ∀ (k : κ), K k ∈ (Finset.image H s).erase (H j)
hK : ∀ (k : κ), K k ≠ H j
hcovers : ⋃ k ∈ Finset.univ, f k • ↑(K k) = Set.univ
k : κ
hcovers' : ¬⋃ i ∈ Finset.filter (fun x => K x = K k) Finset.univ, f i • ↑(K i) = Set.univ
hn' : (Finset.image K Finset.univ).card < n
⊢ ∃ i ∈ s, H i ≠ H j ∧ (H i).FiniteIndex

Proof state:
G : Type u_1
inst✝¹ : Group G
inst✝ : DecidableEq (Subgroup G)
n : ℕ
ih :
  ∀ m < n,
    ∀ {ι : Type u_2} {H : ι → Subgroup G} {g : ι → G} {s : Finset ι},
      ⋃ i ∈ s, g i • ↑(H i) = Set.univ →
        ∀ j ∈ s,
          ⋃ i ∈ Finset.filter (fun x => H x = H j) s, g i • ↑(H i) ≠ Set.univ →
            m = (Finset.image H s).card → ∃ i ∈ s, H i ≠ H j ∧ (H i).FiniteIndex
ι : Type u_2
H : ι → Subgroup G
g : ι → G
s : Finset ι
hcovers : ⋃ i ∈ s, g i • ↑(H i) = Set.univ
j : ι
hj : j ∈ s
hcovers' : ⋃ i ∈ Finset.filter (fun x => H x = H j) s, g i • ↑(H i) ≠ Set.univ
hn : n = (Finset.image H s).card
x : G
hx : ∀ i ∈ s, H i = H j → ↑(g i) ≠ ↑x
y z : G
hz : z ∈ y • ↑(H j)
i : ι
hi : i ∈ s
hmem : ↑(g i) = ↑(x * (y⁻¹ * z))
⊢ ∃ i, ∃ (_ : i ∈ s ∧ H i ≠ H j), z ∈ (y * x⁻¹ * g i) • ↑(H i)

Tactic:
refine ⟨i, ⟨hi, fun hij => hx i hi hij ?_⟩, ?_⟩

Result:
case refine_1
G : Type u_1
inst✝¹ : Group G
inst✝ : DecidableEq (Subgroup G)
n : ℕ
ih :
  ∀ m < n,
    ∀ {ι : Type u_2} {H : ι → Subgroup G} {g : ι → G} {s : Finset ι},
      ⋃ i ∈ s, g i • ↑(H i) = Set.univ →
        ∀ j ∈ s,
          ⋃ i ∈ Finset.filter (fun x => H x = H j) s, g i • ↑(H i) ≠ Set.univ →
            m = (Finset.image H s).card → ∃ i ∈ s, H i ≠ H j ∧ (H i).FiniteIndex
ι : Type u_2
H : ι → Subgroup G
g : ι → G
s : Finset ι
hcovers : ⋃ i ∈ s, g i • ↑(H i) = Set.univ
j : ι
hj : j ∈ s
hcovers' : ⋃ i ∈ Finset.filter (fun x => H x = H j) s, g i • ↑(H i) ≠ Set.univ
hn : n = (Finset.image H s).card
x : G
hx : ∀ i ∈ s, H i = H j → ↑(g i) ≠ ↑x
y z : G
hz : z ∈ y • ↑(H j)
i : ι
hi : i ∈ s
hmem : ↑(g i) = ↑(x * (y⁻¹ * z))
hij : H i = H j
⊢ ↑(g i) = ↑x

case refine_2
G : Type u_1
inst✝¹ : Group G
inst✝ : DecidableEq (Subgroup G)
n : ℕ
ih :
  ∀ m < n,
    ∀ {ι : Type u_2} {H : ι → Subgroup G} {g : ι → G} {s : Finset ι},
      ⋃ i ∈ s, g i • ↑(H i) = Set.univ →
        ∀ j ∈ s,
          ⋃ i ∈ Finset.filter (fun x => H x = H j) s, g i • ↑(H i) ≠ Set.univ →
            m = (Finset.image H s).card → ∃ i ∈ s, H i ≠ H j ∧ (H i).FiniteIndex
ι : Type u_2
H : ι → Subgroup G
g : ι → G
s : Finset ι
hcovers : ⋃ i ∈ s, g i • ↑(H i) = Set.univ
j : ι
hj : j ∈ s
hcovers' : ⋃ i ∈ Finset.filter (fun x => H x = H j) s, g i • ↑(H i) ≠ Set.univ
hn : n = (Finset.image H s).card
x : G
hx : ∀ i ∈ s, H i = H j → ↑(g i) ≠ ↑x
y z : G
hz : z ∈ y • ↑(H j)
i : ι
hi : i ∈ s
hmem : ↑(g i) = ↑(x * (y⁻¹ * z))
⊢ z ∈ (y * x⁻¹ * g i) • ↑(H i)