Module: Mathlib.Order.RelSeries

Goal state before:
pp: case succ.refine_2
α : Type u_3
inst✝² : PartialOrder α
inst✝¹ : WellFoundedLT α
inst✝ : WellFoundedGT α
n : ℕ
IH :
  ∀ (s : Fin (n + 1) → α) (h : ∀ (i : Fin n), (s i.castSucc, s i.succ) ∈ {(a, b) | a < b}),
    ∃ t i,
      t.toFun ∘ ⇑i = { length := n, toFun := s, step := h }.toFun ∧
        i 0 = 0 ∧ i (Fin.last { length := n, toFun := s, step := h }.length) = Fin.last t.length
s : Fin (n + 1 + 1) → α
h : ∀ (i : Fin (n + 1)), (s i.castSucc, s i.succ) ∈ {(a, b) | a < b}
t₁ : RelSeries {(a, b) | a ⋖ b}
i : Fin ({ length := n, toFun := s ∘ Fin.castSucc, step := ⋯ }.length + 1) ↪ Fin (t₁.length + 1)
ht : t₁.toFun ∘ ⇑i = { length := n, toFun := s ∘ Fin.castSucc, step := ⋯ }.toFun
hi₁ : i 0 = 0
hi₂ : i (Fin.last { length := n, toFun := s ∘ Fin.castSucc, step := ⋯ }.length) = Fin.last t₁.length
t₂ : ℕ → α
h₁ : t₂ 0 = s (Fin.last n).castSucc
m : ℕ
h₂ : t₂ m = s (Fin.last n).succ
ht₂ : ∀ i < m, t₂ i ⋖ t₂ (i + 1)
t₃ : RelSeries {(a, b) | a ⋖ b} := { length := m, toFun := fun x ↦ t₂ ↑x, step := ⋯ }
H : t₁.last = t₂ 0
⊢ (t₁.smash t₃ H).toFun ∘ ⇑{ toFun := Fin.snoc (Fin.castLE ⋯ ∘ ⇑i) (Fin.last (t₁.smash t₃ H).length), inj' := ⋯ } =
    { length := n + 1, toFun := s, step := h }.toFun

Tactic:
refine funext (Fin.lastCases ?_ fun j ↦ ?_)

Goal state after:
case succ.refine_2.refine_1
α : Type u_3
inst✝² : PartialOrder α
inst✝¹ : WellFoundedLT α
inst✝ : WellFoundedGT α
n : ℕ
IH :
  ∀ (s : Fin (n + 1) → α) (h : ∀ (i : Fin n), (s i.castSucc, s i.succ) ∈ {(a, b) | a < b}),
    ∃ t i,
      t.toFun ∘ ⇑i = { length := n, toFun := s, step := h }.toFun ∧
        i 0 = 0 ∧ i (Fin.last { length := n, toFun := s, step := h }.length) = Fin.last t.length
s : Fin (n + 1 + 1) → α
h : ∀ (i : Fin (n + 1)), (s i.castSucc, s i.succ) ∈ {(a, b) | a < b}
t₁ : RelSeries {(a, b) | a ⋖ b}
i : Fin ({ length := n, toFun := s ∘ Fin.castSucc, step := ⋯ }.length + 1) ↪ Fin (t₁.length + 1)
ht : t₁.toFun ∘ ⇑i = { length := n, toFun := s ∘ Fin.castSucc, step := ⋯ }.toFun
hi₁ : i 0 = 0
hi₂ : i (Fin.last { length := n, toFun := s ∘ Fin.castSucc, step := ⋯ }.length) = Fin.last t₁.length
t₂ : ℕ → α
h₁ : t₂ 0 = s (Fin.last n).castSucc
m : ℕ
h₂ : t₂ m = s (Fin.last n).succ
ht₂ : ∀ i < m, t₂ i ⋖ t₂ (i + 1)
t₃ : RelSeries {(a, b) | a ⋖ b} := { length := m, toFun := fun x ↦ t₂ ↑x, step := ⋯ }
H : t₁.last = t₂ 0
⊢ ((t₁.smash t₃ H).toFun ∘ ⇑{ toFun := Fin.snoc (Fin.castLE ⋯ ∘ ⇑i) (Fin.last (t₁.smash t₃ H).length), inj' := ⋯ })
      (Fin.last { length := n + 1, toFun := s, step := h }.length) =
    { length := n + 1, toFun := s, step := h }.toFun (Fin.last { length := n + 1, toFun := s, step := h }.length)

case succ.refine_2.refine_2
α : Type u_3
inst✝² : PartialOrder α
inst✝¹ : WellFoundedLT α
inst✝ : WellFoundedGT α
n : ℕ
IH :
  ∀ (s : Fin (n + 1) → α) (h : ∀ (i : Fin n), (s i.castSucc, s i.succ) ∈ {(a, b) | a < b}),
    ∃ t i,
      t.toFun ∘ ⇑i = { length := n, toFun := s, step := h }.toFun ∧
        i 0 = 0 ∧ i (Fin.last { length := n, toFun := s, step := h }.length) = Fin.last t.length
s : Fin (n + 1 + 1) → α
h : ∀ (i : Fin (n + 1)), (s i.castSucc, s i.succ) ∈ {(a, b) | a < b}
t₁ : RelSeries {(a, b) | a ⋖ b}
i : Fin ({ length := n, toFun := s ∘ Fin.castSucc, step := ⋯ }.length + 1) ↪ Fin (t₁.length + 1)
ht : t₁.toFun ∘ ⇑i = { length := n, toFun := s ∘ Fin.castSucc, step := ⋯ }.toFun
hi₁ : i 0 = 0
hi₂ : i (Fin.last { length := n, toFun := s ∘ Fin.castSucc, step := ⋯ }.length) = Fin.last t₁.length
t₂ : ℕ → α
h₁ : t₂ 0 = s (Fin.last n).castSucc
m : ℕ
h₂ : t₂ m = s (Fin.last n).succ
ht₂ : ∀ i < m, t₂ i ⋖ t₂ (i + 1)
t₃ : RelSeries {(a, b) | a ⋖ b} := { length := m, toFun := fun x ↦ t₂ ↑x, step := ⋯ }
H : t₁.last = t₂ 0
j : Fin { length := n + 1, toFun := s, step := h }.length
⊢ ((t₁.smash t₃ H).toFun ∘ ⇑{ toFun := Fin.snoc (Fin.castLE ⋯ ∘ ⇑i) (Fin.last (t₁.smash t₃ H).length), inj' := ⋯ })
      j.castSucc =
    { length := n + 1, toFun := s, step := h }.toFun j.castSucc