-- 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:
case 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
this : IsStronglyAtomic αs
a₀ : αs
ha₀ : a₀.fst = 0
ha₀inf : (Ici a₀).Infinite
hfin : ∀ (a : αs), {x | a ⋖ x}.Finite
⊢ ∃ f, ∀ ⦃i j : ℕ⦄ (hij : i ≤ j), π hij (f j) = f i

Tactic:
obtain ⟨f, hf0, hf⟩ := exists_orderEmbedding_covby_of_forall_covby_finite hfin ha₀inf

Result:
case intro.intro.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
this : IsStronglyAtomic αs
a₀ : αs
ha₀ : a₀.fst = 0
ha₀inf : (Ici a₀).Infinite
hfin : ∀ (a : αs), {x | a ⋖ x}.Finite
f : ℕ ↪o αs
hf0 : f 0 = a₀
hf : ∀ (i : ℕ), f i ⋖ f (i + 1)
⊢ ∃ f, ∀ ⦃i j : ℕ⦄ (hij : i ≤ j), π hij (f j) = f i

Proof state:
case intro.intro.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
this : IsStronglyAtomic αs
a₀ : αs
ha₀ : a₀.fst = 0
ha₀inf : (Ici a₀).Infinite
hfin : ∀ (a : αs), {x | a ⋖ x}.Finite
f : ℕ ↪o αs
hf0 : f 0 = a₀
hf : ∀ (i : ℕ), f i ⋖ f (i + 1)
⊢ ∃ f, ∀ ⦃i j : ℕ⦄ (hij : i ≤ j), π hij (f j) = f i

Tactic:
have hr : ∀ i, (f i).1 = i :=
  Nat.rec (by rw [hf0, ha₀]) (fun i ih ↦ by rw [← (hcovby.1 (hf i)).2, ih])

Result:
case intro.intro.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
this : IsStronglyAtomic αs
a₀ : αs
ha₀ : a₀.fst = 0
ha₀inf : (Ici a₀).Infinite
hfin : ∀ (a : αs), {x | a ⋖ x}.Finite
f : ℕ ↪o αs
hf0 : f 0 = a₀
hf : ∀ (i : ℕ), f i ⋖ f (i + 1)
hr : ∀ (i : ℕ), (f i).fst = i
⊢ ∃ 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⟩
⊢ 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✝¹ : PartialOrder αs := PartialOrder.mk ⋯
i : ℕ
b : α i
h : i ≤ i
x✝ :
  ¬⟨i, π h b⟩ = ⟨i, b⟩ ∧
    ∀ (a : ℕ) (b_1 : α a), ⟨i, π h b⟩ ≤ ⟨a, b_1⟩ → ⟨a, b_1⟩ ≤ ⟨i, b⟩ → ¬⟨a, b_1⟩ = ⟨i, π h b⟩ → ⟨a, b_1⟩ = ⟨i, b⟩
hne : ¬⟨i, π h b⟩ = ⟨i, b⟩
h' : ∀ (a : ℕ) (b_1 : α a), ⟨i, π h b⟩ ≤ ⟨a, b_1⟩ → ⟨a, b_1⟩ ≤ ⟨i, b⟩ → ¬⟨a, b_1⟩ = ⟨i, π h b⟩ → ⟨a, b_1⟩ = ⟨i, b⟩
⊢ False

Proof state:
case intro.refine_2
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 j : ℕ
b : α j
h : i ≤ j
⊢ i + 1 = j →
    ¬⟨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⟩

Tactic:
rintro rfl

Result:
case intro.refine_2
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
⊢ ¬⟨i, π h b⟩ = ⟨i + 1, b⟩ ∧
    ∀ (a : ℕ) (b_1 : α a),
      ⟨i, π h b⟩ ≤ ⟨a, b_1⟩ → ⟨a, b_1⟩ ≤ ⟨i + 1, b⟩ → ¬⟨a, b_1⟩ = ⟨i, π h b⟩ → ⟨a, b_1⟩ = ⟨i + 1, b⟩

Proof state:
case intro.refine_2
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
⊢ ¬⟨i, π h b⟩ = ⟨i + 1, b⟩ ∧
    ∀ (a : ℕ) (b_1 : α a),
      ⟨i, π h b⟩ ≤ ⟨a, b_1⟩ → ⟨a, b_1⟩ ≤ ⟨i + 1, b⟩ → ¬⟨a, b_1⟩ = ⟨i, π h b⟩ → ⟨a, b_1⟩ = ⟨i + 1, b⟩

Tactic:
refine ⟨fun h ↦ by apply_fun Sigma.fst at h; simp at h, ?_⟩

Result:
case intro.refine_2
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
⊢ ∀ (a : ℕ) (b_1 : α a), ⟨i, π h b⟩ ≤ ⟨a, b_1⟩ → ⟨a, b_1⟩ ≤ ⟨i + 1, b⟩ → ¬⟨a, b_1⟩ = ⟨i, π h b⟩ → ⟨a, b_1⟩ = ⟨i + 1, b⟩

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
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

Tactic:
contradiction

Result:
no goals