-- Mathlib/Order/WellFoundedSet.lean
theorem subsetProdLex [PartialOrder α] [Preorder β] {s : Set (α ×ₗ β)}
    (hα : ((fun (x : α ×ₗ β) => (ofLex x).1)'' s).IsPWO)
    (hβ : ∀ a, {y | toLex (a, y) ∈ s}.IsPWO) : s.IsPWO

Proof state:
case neg.intro
α : Type u_2
β : Type u_3
inst✝¹ : PartialOrder α
inst✝ : Preorder β
s : Set (Lex (α × β))
hα : ∀ (f : ℕ → α), (∀ (n : ℕ), f n ∈ (fun x => (ofLex x).1) '' s) → ∃ g, Monotone (f ∘ ⇑g)
hβ : ∀ (a : α), {y | toLex (a, y) ∈ s}.IsPWO
f : ℕ → Lex (α × β)
hf : ∀ (n : ℕ), f n ∈ s
g : ℕ ↪o ℕ
hg : Monotone fun n => (ofLex f (g n)).1
hhg : ∀ (n : ℕ), (ofLex f (g 0)).1 ≤ (ofLex f (g n)).1
hc : ¬∃ n, (ofLex f (g 0)).1 < (ofLex f (g n)).1
hhc : ∀ (n : ℕ), (ofLex f (g 0)).1 = (ofLex f (g n)).1
g' : ℕ ↪o ℕ
hg' : Monotone fun n => (ofLex f (g (g' n))).2
⊢ ∃ m n, m < n ∧ (fun x1 x2 => x1 ≤ x2) (f m) (f n)

Tactic:
use (g (g' 0)), (g (g' 1))

Result:
case h
α : Type u_2
β : Type u_3
inst✝¹ : PartialOrder α
inst✝ : Preorder β
s : Set (Lex (α × β))
hα : ∀ (f : ℕ → α), (∀ (n : ℕ), f n ∈ (fun x => (ofLex x).1) '' s) → ∃ g, Monotone (f ∘ ⇑g)
hβ : ∀ (a : α), {y | toLex (a, y) ∈ s}.IsPWO
f : ℕ → Lex (α × β)
hf : ∀ (n : ℕ), f n ∈ s
g : ℕ ↪o ℕ
hg : Monotone fun n => (ofLex f (g n)).1
hhg : ∀ (n : ℕ), (ofLex f (g 0)).1 ≤ (ofLex f (g n)).1
hc : ¬∃ n, (ofLex f (g 0)).1 < (ofLex f (g n)).1
hhc : ∀ (n : ℕ), (ofLex f (g 0)).1 = (ofLex f (g n)).1
g' : ℕ ↪o ℕ
hg' : Monotone fun n => (ofLex f (g (g' n))).2
⊢ g (g' 0) < g (g' 1) ∧ (fun x1 x2 => x1 ≤ x2) (f (g (g' 0))) (f (g (g' 1)))

Proof state:
α : Type u_2
β : Type u_3
inst✝¹ : PartialOrder α
inst✝ : Preorder β
s : Set (Lex (α × β))
hα : ∀ (f : ℕ → α), (∀ (n : ℕ), f n ∈ (fun x => (ofLex x).1) '' s) → ∃ g, Monotone (f ∘ ⇑g)
hβ : ∀ (a : α), {y | toLex (a, y) ∈ s}.IsPWO
f : ℕ → Lex (α × β)
hf : ∀ (n : ℕ), f n ∈ s
g : ℕ ↪o ℕ
hg : Monotone fun n => (ofLex f (g n)).1
hhg : ∀ (n : ℕ), (ofLex f (g 0)).1 ≤ (ofLex f (g n)).1
hc : ∀ (x : ℕ), ¬(ofLex f (g 0)).1 < (ofLex f (g x)).1
n : ℕ
⊢ (ofLex f (g 0)).1 = (ofLex f (g n)).1

Tactic:
exact (hhg n).eq_of_not_lt (hc n)

Result:
no goals

Proof state:
case h.left
α : Type u_2
β : Type u_3
inst✝¹ : PartialOrder α
inst✝ : Preorder β
s : Set (Lex (α × β))
hα : ∀ (f : ℕ → α), (∀ (n : ℕ), f n ∈ (fun x => (ofLex x).1) '' s) → ∃ g, Monotone (f ∘ ⇑g)
hβ : ∀ (a : α), {y | toLex (a, y) ∈ s}.IsPWO
f : ℕ → Lex (α × β)
hf : ∀ (n : ℕ), f n ∈ s
g : ℕ ↪o ℕ
hg : Monotone fun n => (ofLex f (g n)).1
hhg : ∀ (n : ℕ), (ofLex f (g 0)).1 ≤ (ofLex f (g n)).1
n : ℕ
hn : (ofLex f (g 0)).1 < (ofLex f (g n)).1
⊢ g 0 < g n

Tactic:
by_contra hx

Result:
case h.left
α : Type u_2
β : Type u_3
inst✝¹ : PartialOrder α
inst✝ : Preorder β
s : Set (Lex (α × β))
hα : ∀ (f : ℕ → α), (∀ (n : ℕ), f n ∈ (fun x => (ofLex x).1) '' s) → ∃ g, Monotone (f ∘ ⇑g)
hβ : ∀ (a : α), {y | toLex (a, y) ∈ s}.IsPWO
f : ℕ → Lex (α × β)
hf : ∀ (n : ℕ), f n ∈ s
g : ℕ ↪o ℕ
hg : Monotone fun n => (ofLex f (g n)).1
hhg : ∀ (n : ℕ), (ofLex f (g 0)).1 ≤ (ofLex f (g n)).1
n : ℕ
hn : (ofLex f (g 0)).1 < (ofLex f (g n)).1
hx : ¬g 0 < g n
⊢ False

Proof state:
case neg
α : Type u_2
β : Type u_3
inst✝¹ : PartialOrder α
inst✝ : Preorder β
s : Set (Lex (α × β))
hα : ∀ (f : ℕ → α), (∀ (n : ℕ), f n ∈ (fun x => (ofLex x).1) '' s) → ∃ g, Monotone (f ∘ ⇑g)
hβ : ∀ (a : α), {y | toLex (a, y) ∈ s}.IsPWO
f : ℕ → Lex (α × β)
hf : ∀ (n : ℕ), f n ∈ s
g : ℕ ↪o ℕ
hg : Monotone fun n => (ofLex f (g n)).1
hhg : ∀ (n : ℕ), (ofLex f (g 0)).1 ≤ (ofLex f (g n)).1
hc : ¬∃ n, (ofLex f (g 0)).1 < (ofLex f (g n)).1
hhc : ∀ (n : ℕ), (ofLex f (g 0)).1 = (ofLex f (g n)).1
⊢ ∃ m n, m < n ∧ (fun x1 x2 => x1 ≤ x2) (f m) (f n)

Tactic:
obtain ⟨g', hg'⟩ : ∃ g' : ℕ ↪o ℕ, Monotone ((fun n ↦ (ofLex f (g (g' n))).2)) := by
  simp_rw [isPWO_iff_exists_monotone_subseq] at hβ
  apply hβ (ofLex f (g 0)).1 fun n ↦ (ofLex f (g n)).2
  intro n
  rw [hhc n]
  simpa using hf _

Result:
case neg.intro
α : Type u_2
β : Type u_3
inst✝¹ : PartialOrder α
inst✝ : Preorder β
s : Set (Lex (α × β))
hα : ∀ (f : ℕ → α), (∀ (n : ℕ), f n ∈ (fun x => (ofLex x).1) '' s) → ∃ g, Monotone (f ∘ ⇑g)
hβ : ∀ (a : α), {y | toLex (a, y) ∈ s}.IsPWO
f : ℕ → Lex (α × β)
hf : ∀ (n : ℕ), f n ∈ s
g : ℕ ↪o ℕ
hg : Monotone fun n => (ofLex f (g n)).1
hhg : ∀ (n : ℕ), (ofLex f (g 0)).1 ≤ (ofLex f (g n)).1
hc : ¬∃ n, (ofLex f (g 0)).1 < (ofLex f (g n)).1
hhc : ∀ (n : ℕ), (ofLex f (g 0)).1 = (ofLex f (g n)).1
g' : ℕ ↪o ℕ
hg' : Monotone fun n => (ofLex f (g (g' n))).2
⊢ ∃ m n, m < n ∧ (fun x1 x2 => x1 ≤ x2) (f m) (f n)

Proof state:
case intro
α : Type u_2
β : Type u_3
inst✝¹ : PartialOrder α
inst✝ : Preorder β
s : Set (Lex (α × β))
hα : ∀ (f : ℕ → α), (∀ (n : ℕ), f n ∈ (fun x => (ofLex x).1) '' s) → ∃ g, Monotone (f ∘ ⇑g)
hβ : ∀ (a : α), {y | toLex (a, y) ∈ s}.IsPWO
f : ℕ → Lex (α × β)
hf : ∀ (n : ℕ), f n ∈ s
g : ℕ ↪o ℕ
hg : Monotone fun n => (ofLex f (g n)).1
hhg : ∀ (n : ℕ), (ofLex f (g 0)).1 ≤ (ofLex f (g n)).1
⊢ ∃ m n, m < n ∧ (fun x1 x2 => x1 ≤ x2) (f m) (f n)

Tactic:
by_cases hc : ∃ n, (ofLex f (g 0)).1 < (ofLex f (g n)).1

Result:
case pos
α : Type u_2
β : Type u_3
inst✝¹ : PartialOrder α
inst✝ : Preorder β
s : Set (Lex (α × β))
hα : ∀ (f : ℕ → α), (∀ (n : ℕ), f n ∈ (fun x => (ofLex x).1) '' s) → ∃ g, Monotone (f ∘ ⇑g)
hβ : ∀ (a : α), {y | toLex (a, y) ∈ s}.IsPWO
f : ℕ → Lex (α × β)
hf : ∀ (n : ℕ), f n ∈ s
g : ℕ ↪o ℕ
hg : Monotone fun n => (ofLex f (g n)).1
hhg : ∀ (n : ℕ), (ofLex f (g 0)).1 ≤ (ofLex f (g n)).1
hc : ∃ n, (ofLex f (g 0)).1 < (ofLex f (g n)).1
⊢ ∃ m n, m < n ∧ (fun x1 x2 => x1 ≤ x2) (f m) (f n)

case neg
α : Type u_2
β : Type u_3
inst✝¹ : PartialOrder α
inst✝ : Preorder β
s : Set (Lex (α × β))
hα : ∀ (f : ℕ → α), (∀ (n : ℕ), f n ∈ (fun x => (ofLex x).1) '' s) → ∃ g, Monotone (f ∘ ⇑g)
hβ : ∀ (a : α), {y | toLex (a, y) ∈ s}.IsPWO
f : ℕ → Lex (α × β)
hf : ∀ (n : ℕ), f n ∈ s
g : ℕ ↪o ℕ
hg : Monotone fun n => (ofLex f (g n)).1
hhg : ∀ (n : ℕ), (ofLex f (g 0)).1 ≤ (ofLex f (g n)).1
hc : ¬∃ n, (ofLex f (g 0)).1 < (ofLex f (g n)).1
⊢ ∃ m n, m < n ∧ (fun x1 x2 => x1 ≤ x2) (f m) (f n)

Proof state:
α : Type u_2
β : Type u_3
inst✝¹ : PartialOrder α
inst✝ : Preorder β
s : Set (Lex (α × β))
hα : ∀ (f : ℕ → α), (∀ (n : ℕ), f n ∈ (fun x => (ofLex x).1) '' s) → ∃ g, Monotone (f ∘ ⇑g)
f : ℕ → Lex (α × β)
hf : ∀ (n : ℕ), f n ∈ s
g : ℕ ↪o ℕ
hg : Monotone fun n => (ofLex f (g n)).1
hhg : ∀ (n : ℕ), (ofLex f (g 0)).1 ≤ (ofLex f (g n)).1
hc : ¬∃ n, (ofLex f (g 0)).1 < (ofLex f (g n)).1
hhc : ∀ (n : ℕ), (ofLex f (g 0)).1 = (ofLex f (g n)).1
hβ : ∀ (a : α) (f : ℕ → β), (∀ (n : ℕ), f n ∈ {y | toLex (a, y) ∈ s}) → ∃ g, Monotone (f ∘ ⇑g)
n : ℕ
⊢ (ofLex f (g n)).2 ∈ {y | toLex ((ofLex f (g 0)).1, y) ∈ s}

Tactic:
rw [hhc n]

Result:
α : Type u_2
β : Type u_3
inst✝¹ : PartialOrder α
inst✝ : Preorder β
s : Set (Lex (α × β))
hα : ∀ (f : ℕ → α), (∀ (n : ℕ), f n ∈ (fun x => (ofLex x).1) '' s) → ∃ g, Monotone (f ∘ ⇑g)
f : ℕ → Lex (α × β)
hf : ∀ (n : ℕ), f n ∈ s
g : ℕ ↪o ℕ
hg : Monotone fun n => (ofLex f (g n)).1
hhg : ∀ (n : ℕ), (ofLex f (g 0)).1 ≤ (ofLex f (g n)).1
hc : ¬∃ n, (ofLex f (g 0)).1 < (ofLex f (g n)).1
hhc : ∀ (n : ℕ), (ofLex f (g 0)).1 = (ofLex f (g n)).1
hβ : ∀ (a : α) (f : ℕ → β), (∀ (n : ℕ), f n ∈ {y | toLex (a, y) ∈ s}) → ∃ g, Monotone (f ∘ ⇑g)
n : ℕ
⊢ (ofLex f (g n)).2 ∈ {y | toLex ((ofLex f (g n)).1, y) ∈ s}