-- Mathlib/Order/KonigLemma.lean
theorem exists_seq_forall_proj_of_forall_finite {α : ℕ → Type*} [Finite (α 0)] [∀ i, Nonempty (α i)]
    (π : {i j : ℕ} → (hij : i ≤ j) → α j → α i)
    (π_refl : ∀ ⦃i⦄ (a : α i), π rfl.le a = a)
    (π_trans : ∀ ⦃i j k⦄ (hij : i ≤ j) (hjk : j ≤ k) a, π hij (π hjk a) = π (hij.trans hjk) a)
    (hfin : ∀ i a, {b : α (i+1) | π (Nat.le_add_right i 1) b = a}.Finite) :
    ∃ f : (i : ℕ) → α i, ∀ ⦃i j⦄ (hij : i ≤ j), π hij (f j) = f i

Proof state:
α : ℕ → Type u_1
inst✝¹ : Finite (α 0)
inst✝ : ∀ (i : ℕ), Nonempty (α i)
π : {i j : ℕ} → i ≤ j → α j → α i
π_refl : ∀ ⦃i : ℕ⦄ (a : α i), π ⋯ a = a
π_trans : ∀ ⦃i j k : ℕ⦄ (hij : i ≤ j) (hjk : j ≤ k) (a : α k), π hij (π hjk a) = π ⋯ a
hfin : ∀ (i : ℕ) (a : α i), {b | π ⋯ b = a}.Finite
αs : Type u_1 := (i : ℕ) × α i
x✝¹ : PartialOrder αs := PartialOrder.mk ⋯
i j : ℕ
b : α j
h : i ≤ j
x✝ :
  ¬⟨i, π h b⟩ = ⟨j, b⟩ ∧
    ∀ (a : ℕ) (b_1 : α a), ⟨i, π h b⟩ ≤ ⟨a, b_1⟩ → ⟨a, b_1⟩ ≤ ⟨j, b⟩ → ¬⟨a, b_1⟩ = ⟨i, π h b⟩ → ⟨a, b_1⟩ = ⟨j, b⟩
hne : ¬⟨i, π h b⟩ = ⟨j, b⟩
h' : ∀ (a : ℕ) (b_1 : α a), ⟨i, π h b⟩ ≤ ⟨a, b_1⟩ → ⟨a, b_1⟩ ≤ ⟨j, b⟩ → ¬⟨a, b_1⟩ = ⟨i, π h b⟩ → ⟨a, b_1⟩ = ⟨j, b⟩
hle' : i + 1 ≤ j
⊢ ⟨i, π h b⟩.fst ≤ ⟨i + 1, π hle' b⟩.fst

Tactic:
simp

Result:
no goals

Proof state:
α : ℕ → Type u_1
inst✝¹ : Finite (α 0)
inst✝ : ∀ (i : ℕ), Nonempty (α i)
π : {i j : ℕ} → i ≤ j → α j → α i
π_refl : ∀ ⦃i : ℕ⦄ (a : α i), π ⋯ a = a
π_trans : ∀ ⦃i j k : ℕ⦄ (hij : i ≤ j) (hjk : j ≤ k) (a : α k), π hij (π hjk a) = π ⋯ a
hfin : ∀ (i : ℕ) (a : α i), {b | π ⋯ b = a}.Finite
αs : Type u_1 := (i : ℕ) × α i
x✝ : PartialOrder αs := PartialOrder.mk ⋯
hcovby : ∀ {a b : αs}, a ⋖ b ↔ a ≤ b ∧ a.fst + 1 = b.fst
i : ℕ
a : α i
j : ℕ
b : α j
hne : ⟨i, a⟩ ≠ ⟨j, b⟩
hij : i ≤ j
h2 : π hij b = a
hle : i + 1 ≤ j
⊢ π ⋯ ⟨i + 1, π hle b⟩.snd = ⟨i, a⟩.snd

Tactic:
rw [π_trans, ← h2]

Result:
no goals

Proof state:
case intro
α : ℕ → Type u_1
inst✝¹ : Finite (α 0)
inst✝ : ∀ (i : ℕ), Nonempty (α i)
π : {i j : ℕ} → i ≤ j → α j → α i
π_refl : ∀ ⦃i : ℕ⦄ (a : α i), π ⋯ a = a
π_trans : ∀ ⦃i j k : ℕ⦄ (hij : i ≤ j) (hjk : j ≤ k) (a : α k), π hij (π hjk a) = π ⋯ a
hfin : ∀ (i : ℕ) (a : α i), {b | π ⋯ b = a}.Finite
αs : Type u_1 := (i : ℕ) × α i
x✝ : PartialOrder αs := PartialOrder.mk ⋯
hcovby : ∀ {a b : αs}, a ⋖ b ↔ a ≤ b ∧ a.fst + 1 = b.fst
this : IsStronglyAtomic αs
a₀ : α 0
ha₀ : Infinite ↑((fun a => π ⋯ a.snd) ⁻¹' {a₀})
⊢ ∀ (a : ℕ) (b : α a), π ⋯ b = a₀ → ⟨0, a₀⟩ ≤ ⟨a, b⟩

Tactic:
exact fun i x h ↦ ⟨zero_le i, h⟩

Result:
no goals

Proof state:
α : ℕ → Type u_1
inst✝¹ : Finite (α 0)
inst✝ : ∀ (i : ℕ), Nonempty (α i)
π : {i j : ℕ} → i ≤ j → α j → α i
π_refl : ∀ ⦃i : ℕ⦄ (a : α i), π ⋯ a = a
π_trans : ∀ ⦃i j k : ℕ⦄ (hij : i ≤ j) (hjk : j ≤ k) (a : α k), π hij (π hjk a) = π ⋯ a
hfin : ∀ (i : ℕ) (a : α i), {b | π ⋯ b = a}.Finite
αs : Type u_1 := (i : ℕ) × α i
x✝ : PartialOrder αs := PartialOrder.mk ⋯
hcovby : ∀ {a b : αs}, a ⋖ b ↔ a ≤ b ∧ a.fst + 1 = b.fst
i : ℕ
a : α i
j : ℕ
b : α j
hne : ⟨i, a⟩ ≠ ⟨j, b⟩
hij : i ≤ j
h2 : π hij b = a
hle : i + 1 ≤ j
⊢ ⟨i, a⟩.fst + 1 = ⟨i + 1, π hle b⟩.fst

Tactic:
simp

Result:
no goals

Proof state:
α : ℕ → Type u_1
inst✝¹ : Finite (α 0)
inst✝ : ∀ (i : ℕ), Nonempty (α i)
π : {i j : ℕ} → i ≤ j → α j → α i
π_refl : ∀ ⦃i : ℕ⦄ (a : α i), π ⋯ a = a
π_trans : ∀ ⦃i j k : ℕ⦄ (hij : i ≤ j) (hjk : j ≤ k) (a : α k), π hij (π hjk a) = π ⋯ a
hfin : ∀ (i : ℕ) (a : α i), {b | π ⋯ b = a}.Finite
αs : Type u_1 := (i : ℕ) × α i
x✝ : PartialOrder αs := PartialOrder.mk ⋯
i : ℕ
b : α (i + 1)
h : i ≤ i + 1
j : ℕ
hij : i ≤ j
hji : j ≤ i + 1
hcb : π hij (π hji b) = π h b
hne : ¬⟨j, π hji b⟩ = ⟨i, π h b⟩
⊢ i ≠ j

Tactic:
rintro rfl

Result:
α : ℕ → Type u_1
inst✝¹ : Finite (α 0)
inst✝ : ∀ (i : ℕ), Nonempty (α i)
π : {i j : ℕ} → i ≤ j → α j → α i
π_refl : ∀ ⦃i : ℕ⦄ (a : α i), π ⋯ a = a
π_trans : ∀ ⦃i j k : ℕ⦄ (hij : i ≤ j) (hjk : j ≤ k) (a : α k), π hij (π hjk a) = π ⋯ a
hfin : ∀ (i : ℕ) (a : α i), {b | π ⋯ b = a}.Finite
αs : Type u_1 := (i : ℕ) × α i
x✝ : PartialOrder αs := PartialOrder.mk ⋯
i : ℕ
b : α (i + 1)
h : i ≤ i + 1
hij : i ≤ i
hji : i ≤ i + 1
hcb : π hij (π hji b) = π h b
hne : ¬⟨i, π hji b⟩ = ⟨i, π h b⟩
⊢ False

Proof state:
α : ℕ → Type u_1
inst✝¹ : Finite (α 0)
inst✝ : ∀ (i : ℕ), Nonempty (α i)
π : {i j : ℕ} → i ≤ j → α j → α i
π_refl : ∀ ⦃i : ℕ⦄ (a : α i), π ⋯ a = a
π_trans : ∀ ⦃i j k : ℕ⦄ (hij : i ≤ j) (hjk : j ≤ k) (a : α k), π hij (π hjk a) = π ⋯ a
hfin : ∀ (i : ℕ) (a : α i), {b | π ⋯ b = a}.Finite
αs : Type u_1 := (i : ℕ) × α i
x✝ : PartialOrder αs := PartialOrder.mk ⋯
hcovby : ∀ {a b : αs}, a ⋖ b ↔ a ≤ b ∧ a.fst + 1 = b.fst
⊢ ∃ f, ∀ ⦃i j : ℕ⦄ (hij : i ≤ j), π hij (f j) = f i

Tactic:
have : IsStronglyAtomic αs := by
  simp_rw [isStronglyAtomic_iff, lt_iff_le_and_ne, hcovby]
  rintro ⟨i, a⟩ ⟨j, b⟩ ⟨⟨hij : i ≤ j, h2 : π hij b = a⟩, hne⟩
  have hle : i + 1 ≤ j := hij.lt_of_ne (by rintro rfl; simp [← h2, π_refl] at hne)
  exact ⟨⟨_, π hle b⟩, ⟨⟨by simp, by rw [π_trans, ← h2]⟩, by simp⟩, ⟨hle, by simp⟩⟩

Result:
α : ℕ → Type u_1
inst✝¹ : Finite (α 0)
inst✝ : ∀ (i : ℕ), Nonempty (α i)
π : {i j : ℕ} → i ≤ j → α j → α i
π_refl : ∀ ⦃i : ℕ⦄ (a : α i), π ⋯ a = a
π_trans : ∀ ⦃i j k : ℕ⦄ (hij : i ≤ j) (hjk : j ≤ k) (a : α k), π hij (π hjk a) = π ⋯ a
hfin : ∀ (i : ℕ) (a : α i), {b | π ⋯ b = a}.Finite
αs : Type u_1 := (i : ℕ) × α i
x✝ : PartialOrder αs := PartialOrder.mk ⋯
hcovby : ∀ {a b : αs}, a ⋖ b ↔ a ≤ b ∧ a.fst + 1 = b.fst
this : IsStronglyAtomic αs
⊢ ∃ f, ∀ ⦃i j : ℕ⦄ (hij : i ≤ j), π hij (f j) = f i

Proof state:
α : ℕ → Type u_1
inst✝¹ : Finite (α 0)
inst✝ : ∀ (i : ℕ), Nonempty (α i)
π : {i j : ℕ} → i ≤ j → α j → α i
π_refl : ∀ ⦃i : ℕ⦄ (a : α i), π ⋯ a = a
π_trans : ∀ ⦃i j k : ℕ⦄ (hij : i ≤ j) (hjk : j ≤ k) (a : α k), π hij (π hjk a) = π ⋯ a
hfin : ∀ (i : ℕ) (a : α i), {b | π ⋯ b = a}.Finite
αs : Type u_1 := (i : ℕ) × α i
x✝¹ : PartialOrder αs := PartialOrder.mk ⋯
i j : ℕ
b : α j
h : i ≤ j
x✝ :
  ¬⟨i, π h b⟩ = ⟨j, b⟩ ∧
    ∀ (a : ℕ) (b_1 : α a), ⟨i, π h b⟩ ≤ ⟨a, b_1⟩ → ⟨a, b_1⟩ ≤ ⟨j, b⟩ → ¬⟨a, b_1⟩ = ⟨i, π h b⟩ → ⟨a, b_1⟩ = ⟨j, b⟩
hne : ¬⟨i, π h b⟩ = ⟨j, b⟩
h' : ∀ (a : ℕ) (b_1 : α a), ⟨i, π h b⟩ ≤ ⟨a, b_1⟩ → ⟨a, b_1⟩ ≤ ⟨j, b⟩ → ¬⟨a, b_1⟩ = ⟨i, π h b⟩ → ⟨a, b_1⟩ = ⟨j, b⟩
hle' : i + 1 ≤ j
⊢ π hle' ⟨j, b⟩.snd = ⟨i + 1, π hle' b⟩.snd

Tactic:
simp

Result:
no goals

Proof state:
α : ℕ → Type u_1
inst✝¹ : Finite (α 0)
inst✝ : ∀ (i : ℕ), Nonempty (α i)
π : {i j : ℕ} → i ≤ j → α j → α i
π_refl : ∀ ⦃i : ℕ⦄ (a : α i), π ⋯ a = a
π_trans : ∀ ⦃i j k : ℕ⦄ (hij : i ≤ j) (hjk : j ≤ k) (a : α k), π hij (π hjk a) = π ⋯ a
hfin : ∀ (i : ℕ) (a : α i), {b | π ⋯ b = a}.Finite
αs : Type u_1 := (i : ℕ) × α i
x✝ : PartialOrder αs := PartialOrder.mk ⋯
hcovby : ∀ {a b : αs}, a ⋖ b ↔ a ≤ b ∧ a.fst + 1 = b.fst
⊢ ∀ (a b : αs), a ≤ b ∧ a ≠ b → ∃ x, (a ≤ x ∧ a.fst + 1 = x.fst) ∧ x ≤ b

Tactic:
rintro ⟨i, a⟩ ⟨j, b⟩ ⟨⟨hij : i ≤ j, h2 : π hij b = a⟩, hne⟩

Result:
case mk.mk.intro.intro
α : ℕ → Type u_1
inst✝¹ : Finite (α 0)
inst✝ : ∀ (i : ℕ), Nonempty (α i)
π : {i j : ℕ} → i ≤ j → α j → α i
π_refl : ∀ ⦃i : ℕ⦄ (a : α i), π ⋯ a = a
π_trans : ∀ ⦃i j k : ℕ⦄ (hij : i ≤ j) (hjk : j ≤ k) (a : α k), π hij (π hjk a) = π ⋯ a
hfin : ∀ (i : ℕ) (a : α i), {b | π ⋯ b = a}.Finite
αs : Type u_1 := (i : ℕ) × α i
x✝ : PartialOrder αs := PartialOrder.mk ⋯
hcovby : ∀ {a b : αs}, a ⋖ b ↔ a ≤ b ∧ a.fst + 1 = b.fst
i : ℕ
a : α i
j : ℕ
b : α j
hne : ⟨i, a⟩ ≠ ⟨j, b⟩
hij : i ≤ j
h2 : π hij b = a
⊢ ∃ x, (⟨i, a⟩ ≤ x ∧ ⟨i, a⟩.fst + 1 = x.fst) ∧ x ≤ ⟨j, b⟩