-- Mathlib/Computability/TMToPartrec.lean
theorem splitAtPred_eq {α} (p : α → Bool) :
    ∀ L l₁ o l₂,
      (∀ x ∈ l₁, p x = false) →
        Option.elim' (L = l₁ ∧ l₂ = []) (fun a => p a = true ∧ L = l₁ ++ a::l₂) o →
          splitAtPred p L = (l₁, o, l₂)
  | [], _, none, _, _, ⟨rfl, rfl⟩ => rfl
  | [], l₁, some o, l₂, _, ⟨_, h₃⟩ => by simp at h₃
  | a :: L, l₁, o, l₂, h₁, h₂ => by
    rw [splitAtPred]
    have IH := splitAtPred_eq p L
    rcases o with - | o
    · rcases l₁ with - | ⟨a', l₁⟩ <;> rcases h₂ with ⟨⟨⟩, rfl⟩
      rw [h₁ a (List.Mem.head _), cond, IH L none [] _ ⟨rfl, rfl⟩]
      exact fun x h => h₁ x (List.Mem.tail _ h)
    · rcases l₁ with - | ⟨a', l₁⟩ <;> rcases h₂ with ⟨h₂, ⟨⟩⟩
      · rw [h₂, cond]
      rw [h₁ a (List.Mem.head _), cond, IH l₁ (some o) l₂ _ ⟨h₂, _⟩] <;> try rfl
      exact fun x h => h₁ x (List.Mem.tail _ h)

Proof state:
α : Type u_1
p : α → Bool
a : α
L l₁ : List α
o : Option α
l₂ : List α
h₁ : ∀ x ∈ l₁, p x = false
h₂ : Option.elim' (a :: L = l₁ ∧ l₂ = []) (fun a_1 => p a_1 = true ∧ a :: L = l₁ ++ a_1 :: l₂) o
⊢ splitAtPred p (a :: L) = (l₁, o, l₂)

Tactic:
rw [splitAtPred]

Result:
α : Type u_1
p : α → Bool
a : α
L l₁ : List α
o : Option α
l₂ : List α
h₁ : ∀ x ∈ l₁, p x = false
h₂ : Option.elim' (a :: L = l₁ ∧ l₂ = []) (fun a_1 => p a_1 = true ∧ a :: L = l₁ ++ a_1 :: l₂) o
⊢ (bif p a then ([], some a, L)
    else
      match splitAtPred p L with
      | (l₁, o, l₂) => (a :: l₁, o, l₂)) =
    (l₁, o, l₂)

Proof state:
case some
α : Type u_1
p : α → Bool
a : α
L l₁ l₂ : List α
h₁ : ∀ x ∈ l₁, p x = false
IH :
  ∀ (l₁ : List α) (o : Option α) (l₂ : List α),
    (∀ x ∈ l₁, p x = false) →
      Option.elim' (L = l₁ ∧ l₂ = []) (fun a => p a = true ∧ L = l₁ ++ a :: l₂) o → splitAtPred p L = (l₁, o, l₂)
o : α
h₂ : Option.elim' (a :: L = l₁ ∧ l₂ = []) (fun a_1 => p a_1 = true ∧ a :: L = l₁ ++ a_1 :: l₂) (some o)
⊢ (bif p a then ([], some a, L)
    else
      match splitAtPred p L with
      | (l₁, o, l₂) => (a :: l₁, o, l₂)) =
    (l₁, some o, l₂)

Tactic:
rcases l₁ with - | ⟨a', l₁⟩ <;> rcases h₂ with ⟨h₂, ⟨⟩⟩

Result:
case some.nil.intro.refl
α : Type u_1
p : α → Bool
a : α
L : List α
IH :
  ∀ (l₁ : List α) (o : Option α) (l₂ : List α),
    (∀ x ∈ l₁, p x = false) →
      Option.elim' (L = l₁ ∧ l₂ = []) (fun a => p a = true ∧ L = l₁ ++ a :: l₂) o → splitAtPred p L = (l₁, o, l₂)
h₁ : ∀ x ∈ [], p x = false
h₂ : p a = true
⊢ (bif p a then ([], some a, L)
    else
      match splitAtPred p L with
      | (l₁, o, l₂) => (a :: l₁, o, l₂)) =
    ([], some a, L)

case some.cons.intro.refl
α : Type u_1
p : α → Bool
a : α
l₂ : List α
o : α
l₁ : List α
h₂ : p o = true
h₁ : ∀ x ∈ a :: l₁, p x = false
IH :
  ∀ (l₁_1 : List α) (o_1 : Option α) (l₂_1 : List α),
    (∀ x ∈ l₁_1, p x = false) →
      Option.elim' (l₁.append (o :: l₂) = l₁_1 ∧ l₂_1 = [])
          (fun a => p a = true ∧ l₁.append (o :: l₂) = l₁_1 ++ a :: l₂_1) o_1 →
        splitAtPred p (l₁.append (o :: l₂)) = (l₁_1, o_1, l₂_1)
⊢ (bif p a then ([], some a, l₁.append (o :: l₂))
    else
      match splitAtPred p (l₁.append (o :: l₂)) with
      | (l₁, o, l₂) => (a :: l₁, o, l₂)) =
    (a :: l₁, some o, l₂)

Proof state:
α : Type u_1
p : α → Bool
l₁ : List α
o : α
l₂ : List α
x✝ : ∀ x ∈ l₁, p x = false
left✝ : p o = true
h₃ : [] = l₁ ++ o :: l₂
⊢ splitAtPred p [] = (l₁, some o, l₂)

Tactic:
simp at h₃

Result:
no goals

Proof state:
case some.nil.intro.refl
α : Type u_1
p : α → Bool
a : α
L : List α
IH :
  ∀ (l₁ : List α) (o : Option α) (l₂ : List α),
    (∀ x ∈ l₁, p x = false) →
      Option.elim' (L = l₁ ∧ l₂ = []) (fun a => p a = true ∧ L = l₁ ++ a :: l₂) o → splitAtPred p L = (l₁, o, l₂)
h₁ : ∀ x ∈ [], p x = false
h₂ : p a = true
⊢ (bif p a then ([], some a, L)
    else
      match splitAtPred p L with
      | (l₁, o, l₂) => (a :: l₁, o, l₂)) =
    ([], some a, L)

Tactic:
rw [h₂, cond]

Result:
no goals