case inl
α : Type u_1
sel : Set α → Option α
n₁ n₂ : ℕ
a : α
s : Set α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
h₁ : enumerate sel s n₁ = some a
h₂ : enumerate sel s n₂ = some a
hn : n₁ ≤ n₂
⊢ n₁ = n₂

case inr
α : Type u_1
sel : Set α → Option α
n₁ n₂ : ℕ
a : α
s : Set α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
h₁ : enumerate sel s n₁ = some a
h₂ : enumerate sel s n₂ = some a
hn : n₂ ≤ n₁
⊢ n₁ = n₂
α : Type u_1
sel : Set α → Option α
n₁ n₂ : ℕ
a : α
s : Set α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
h₁ : enumerate sel s n₁ = some a
h₂ : enumerate sel s n₂ = some a
⊢ n₁ = n₂
rcases le_total n₁ n₂ with (hn|hn)
case inl
α : Type u_1
sel : Set α → Option α
n₁ n₂ : ℕ
a : α
s : Set α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
h₁ : enumerate sel s n₁ = some a
h₂ : enumerate sel s n₂ = some a
hn : n₁ ≤ n₂
⊢ n₁ = n₂

case inr
α : Type u_1
sel : Set α → Option α
n₂ n₁ : ℕ
a : α
s : Set α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
h₂ : enumerate sel s n₂ = some a
h₁ : enumerate sel s n₁ = some a
hn : n₁ ≤ n₂
⊢ n₂ = n₁
case inl
α : Type u_1
sel : Set α → Option α
n₁ n₂ : ℕ
a : α
s : Set α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
h₁ : enumerate sel s n₁ = some a
h₂ : enumerate sel s n₂ = some a
hn : n₁ ≤ n₂
⊢ n₁ = n₂

case inr
α : Type u_1
sel : Set α → Option α
n₁ n₂ : ℕ
a : α
s : Set α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
h₁ : enumerate sel s n₁ = some a
h₂ : enumerate sel s n₂ = some a
hn : n₂ ≤ n₁
⊢ n₁ = n₂
on_goal 2 => swap_var n₁ ↔ n₂, h₁ ↔ h₂
no goals
case inl
α : Type u_1
sel : Set α → Option α
n₁ n₂ : ℕ
a : α
s : Set α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
h₁ : enumerate sel s n₁ = some a
h₂ : enumerate sel s n₂ = some a
hn : n₁ ≤ n₂
⊢ n₁ = n₂

case inr
α : Type u_1
sel : Set α → Option α
n₂ n₁ : ℕ
a : α
s : Set α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
h₂ : enumerate sel s n₂ = some a
h₁ : enumerate sel s n₁ = some a
hn : n₁ ≤ n₂
⊢ n₂ = n₁
all_goals
  rcases Nat.le.dest hn with ⟨m, rfl⟩
  clear hn
  induction n₁ generalizing s
  case zero =>
    cases m
    case zero => rfl
    case succ m =>
      have h' : enumerate sel (s \ {a}) m = some a := by
        simp_all only [enumerate, Nat.zero_eq, Nat.add_eq, zero_add]; exact h₂
      have : a ∈ s \ {a} := enumerate_mem sel h_sel h'
      simp_all [Set.mem_diff_singleton]
  case succ k ih =>
    cases h : sel s
    
    case none =>
      simp_all only [add_comm, self_eq_add_left, Nat.add_succ, enumerate_eq_none_of_sel _ h]
    case some _ =>
      simp_all only [add_comm, self_eq_add_left, enumerate, Option.some.injEq,
                     Nat.add_succ, enumerate._eq_2, Nat.succ.injEq]
      exact ih h₁ h₂
case inr
α : Type u_1
sel : Set α → Option α
n₂ n₁ : ℕ
a : α
s : Set α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
h₂ : enumerate sel s n₂ = some a
h₁ : enumerate sel s n₁ = some a
hn : n₁ ≤ n₂
⊢ n₂ = n₁
case inr
α : Type u_1
sel : Set α → Option α
n₁ n₂ : ℕ
a : α
s : Set α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
h₁ : enumerate sel s n₁ = some a
h₂ : enumerate sel s n₂ = some a
hn : n₂ ≤ n₁
⊢ n₁ = n₂
swap_var n₁ ↔ n₂, h₁ ↔ h₂
case inr
α : Type u_1
sel : Set α → Option α
n₂ n₁ : ℕ
a : α
s : Set α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
h₂ : enumerate sel s n₂ = some a
h₁ : enumerate sel s n₁ = some a
hn : n₁ ≤ n₂
⊢ n₂ = n₁
case inr
α : Type u_1
sel : Set α → Option α
n₁ n₂ : ℕ
a : α
s : Set α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
h₁ : enumerate sel s n₁ = some a
h₂ : enumerate sel s n₂ = some a
hn : n₂ ≤ n₁
⊢ n₁ = n₂
swap_var n₁ ↔ n₂, h₁ ↔ h₂
case inr.intro
α : Type u_1
sel : Set α → Option α
n₁ : ℕ
a : α
s : Set α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
h₁ : enumerate sel s n₁ = some a
m : ℕ
h₂ : enumerate sel s (n₁ + m) = some a
hn : n₁ ≤ n₁ + m
⊢ n₁ + m = n₁
case inr
α : Type u_1
sel : Set α → Option α
n₂ n₁ : ℕ
a : α
s : Set α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
h₂ : enumerate sel s n₂ = some a
h₁ : enumerate sel s n₁ = some a
hn : n₁ ≤ n₂
⊢ n₂ = n₁
rcases Nat.le.dest hn with ⟨m, rfl⟩
case inr.intro
α : Type u_1
sel : Set α → Option α
n₁ : ℕ
a : α
s : Set α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
h₁ : enumerate sel s n₁ = some a
m : ℕ
h₂ : enumerate sel s (n₁ + m) = some a
⊢ n₁ + m = n₁
case inr.intro
α : Type u_1
sel : Set α → Option α
n₁ : ℕ
a : α
s : Set α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
h₁ : enumerate sel s n₁ = some a
m : ℕ
h₂ : enumerate sel s (n₁ + m) = some a
hn : n₁ ≤ n₁ + m
⊢ n₁ + m = n₁
clear hn
case inr.intro.zero
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
m : ℕ
s : Set α
h₁ : enumerate sel s Nat.zero = some a
h₂ : enumerate sel s (Nat.zero + m) = some a
⊢ Nat.zero + m = Nat.zero

case inr.intro.succ
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
m n✝ : ℕ
n_ih✝ : ∀ {s : Set α}, enumerate sel s n✝ = some a → enumerate sel s (n✝ + m) = some a → n✝ + m = n✝
s : Set α
h₁ : enumerate sel s (Nat.succ n✝) = some a
h₂ : enumerate sel s (Nat.succ n✝ + m) = some a
⊢ Nat.succ n✝ + m = Nat.succ n✝
case inr.intro
α : Type u_1
sel : Set α → Option α
n₁ : ℕ
a : α
s : Set α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
h₁ : enumerate sel s n₁ = some a
m : ℕ
h₂ : enumerate sel s (n₁ + m) = some a
⊢ n₁ + m = n₁
induction n₁ generalizing s
case inr.intro.succ
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
m n✝ : ℕ
n_ih✝ : ∀ {s : Set α}, enumerate sel s n✝ = some a → enumerate sel s (n✝ + m) = some a → n✝ + m = n✝
s : Set α
h₁ : enumerate sel s (Nat.succ n✝) = some a
h₂ : enumerate sel s (Nat.succ n✝ + m) = some a
⊢ Nat.succ n✝ + m = Nat.succ n✝
case inr.intro.zero
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
m : ℕ
s : Set α
h₁ : enumerate sel s Nat.zero = some a
h₂ : enumerate sel s (Nat.zero + m) = some a
⊢ Nat.zero + m = Nat.zero

case inr.intro.succ
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
m n✝ : ℕ
n_ih✝ : ∀ {s : Set α}, enumerate sel s n✝ = some a → enumerate sel s (n✝ + m) = some a → n✝ + m = n✝
s : Set α
h₁ : enumerate sel s (Nat.succ n✝) = some a
h₂ : enumerate sel s (Nat.succ n✝ + m) = some a
⊢ Nat.succ n✝ + m = Nat.succ n✝
case zero =>
  cases m
  case zero => rfl
  case succ m =>
    have h' : enumerate sel (s \ {a}) m = some a := by
      simp_all only [enumerate, Nat.zero_eq, Nat.add_eq, zero_add]; exact h₂
    have : a ∈ s \ {a} := enumerate_mem sel h_sel h'
    simp_all [Set.mem_diff_singleton]
no goals
case inr.intro.succ
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
m n✝ : ℕ
n_ih✝ : ∀ {s : Set α}, enumerate sel s n✝ = some a → enumerate sel s (n✝ + m) = some a → n✝ + m = n✝
s : Set α
h₁ : enumerate sel s (Nat.succ n✝) = some a
h₂ : enumerate sel s (Nat.succ n✝ + m) = some a
⊢ Nat.succ n✝ + m = Nat.succ n✝
case succ k ih =>
  cases h : sel s
  
  case none =>
    simp_all only [add_comm, self_eq_add_left, Nat.add_succ, enumerate_eq_none_of_sel _ h]
  case some _ =>
    simp_all only [add_comm, self_eq_add_left, enumerate, Option.some.injEq,
                   Nat.add_succ, enumerate._eq_2, Nat.succ.injEq]
    exact ih h₁ h₂
case zero
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
s : Set α
h₁ : enumerate sel s Nat.zero = some a
h₂ : enumerate sel s (Nat.zero + Nat.zero) = some a
⊢ Nat.zero + Nat.zero = Nat.zero

case succ
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
s : Set α
h₁ : enumerate sel s Nat.zero = some a
n✝ : ℕ
h₂ : enumerate sel s (Nat.zero + Nat.succ n✝) = some a
⊢ Nat.zero + Nat.succ n✝ = Nat.zero
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
m : ℕ
s : Set α
h₁ : enumerate sel s Nat.zero = some a
h₂ : enumerate sel s (Nat.zero + m) = some a
⊢ Nat.zero + m = Nat.zero
cases m
case succ
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
s : Set α
h₁ : enumerate sel s Nat.zero = some a
n✝ : ℕ
h₂ : enumerate sel s (Nat.zero + Nat.succ n✝) = some a
⊢ Nat.zero + Nat.succ n✝ = Nat.zero
case zero
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
s : Set α
h₁ : enumerate sel s Nat.zero = some a
h₂ : enumerate sel s (Nat.zero + Nat.zero) = some a
⊢ Nat.zero + Nat.zero = Nat.zero

case succ
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
s : Set α
h₁ : enumerate sel s Nat.zero = some a
n✝ : ℕ
h₂ : enumerate sel s (Nat.zero + Nat.succ n✝) = some a
⊢ Nat.zero + Nat.succ n✝ = Nat.zero
case zero => rfl
no goals
case succ
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
s : Set α
h₁ : enumerate sel s Nat.zero = some a
n✝ : ℕ
h₂ : enumerate sel s (Nat.zero + Nat.succ n✝) = some a
⊢ Nat.zero + Nat.succ n✝ = Nat.zero
case succ m =>
  have h' : enumerate sel (s \ {a}) m = some a := by
    simp_all only [enumerate, Nat.zero_eq, Nat.add_eq, zero_add]; exact h₂
  have : a ∈ s \ {a} := enumerate_mem sel h_sel h'
  simp_all [Set.mem_diff_singleton]
no goals
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
s : Set α
h₁ : enumerate sel s Nat.zero = some a
h₂ : enumerate sel s (Nat.zero + Nat.zero) = some a
⊢ Nat.zero + Nat.zero = Nat.zero
rfl
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
s : Set α
h₁ : enumerate sel s Nat.zero = some a
m : ℕ
h₂ : enumerate sel s (Nat.zero + Nat.succ m) = some a
h' : enumerate sel (s \ {a}) m = some a
⊢ Nat.zero + Nat.succ m = Nat.zero
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
s : Set α
h₁ : enumerate sel s Nat.zero = some a
m : ℕ
h₂ : enumerate sel s (Nat.zero + Nat.succ m) = some a
⊢ Nat.zero + Nat.succ m = Nat.zero
have h' : enumerate sel (s \ {a}) m = some a := by
  simp_all only [enumerate, Nat.zero_eq, Nat.add_eq, zero_add]; exact h₂
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
s : Set α
h₁ : enumerate sel s Nat.zero = some a
m : ℕ
h₂ : enumerate sel s (Nat.zero + Nat.succ m) = some a
h' : enumerate sel (s \ {a}) m = some a
this : a ∈ s \ {a}
⊢ Nat.zero + Nat.succ m = Nat.zero
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
s : Set α
h₁ : enumerate sel s Nat.zero = some a
m : ℕ
h₂ : enumerate sel s (Nat.zero + Nat.succ m) = some a
h' : enumerate sel (s \ {a}) m = some a
⊢ Nat.zero + Nat.succ m = Nat.zero
have : a ∈ s \ {a} := enumerate_mem sel h_sel h'
no goals
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
s : Set α
h₁ : enumerate sel s Nat.zero = some a
m : ℕ
h₂ : enumerate sel s (Nat.zero + Nat.succ m) = some a
h' : enumerate sel (s \ {a}) m = some a
this : a ∈ s \ {a}
⊢ Nat.zero + Nat.succ m = Nat.zero
simp_all [Set.mem_diff_singleton]
α : Type u_1
sel : Set α → Option α
a : α
s : Set α
m : ℕ
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
h₁ : sel s = some a
h₂ :
  (do
      let a ← some a
      enumerate sel (s \ {a}) m) =
    some a
⊢ enumerate sel (s \ {a}) m = some a
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
s : Set α
h₁ : enumerate sel s Nat.zero = some a
m : ℕ
h₂ : enumerate sel s (Nat.zero + Nat.succ m) = some a
⊢ enumerate sel (s \ {a}) m = some a
simp_all only [enumerate, Nat.zero_eq, Nat.add_eq, zero_add]
no goals
α : Type u_1
sel : Set α → Option α
a : α
s : Set α
m : ℕ
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
h₁ : sel s = some a
h₂ :
  (do
      let a ← some a
      enumerate sel (s \ {a}) m) =
    some a
⊢ enumerate sel (s \ {a}) m = some a
exact h₂
case none
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
m k : ℕ
ih : ∀ {s : Set α}, enumerate sel s k = some a → enumerate sel s (k + m) = some a → k + m = k
s : Set α
h₁ : enumerate sel s (Nat.succ k) = some a
h₂ : enumerate sel s (Nat.succ k + m) = some a
h : sel s = none
⊢ Nat.succ k + m = Nat.succ k

case some
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
m k : ℕ
ih : ∀ {s : Set α}, enumerate sel s k = some a → enumerate sel s (k + m) = some a → k + m = k
s : Set α
h₁ : enumerate sel s (Nat.succ k) = some a
h₂ : enumerate sel s (Nat.succ k + m) = some a
val✝ : α
h : sel s = some val✝
⊢ Nat.succ k + m = Nat.succ k
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
m k : ℕ
ih : ∀ {s : Set α}, enumerate sel s k = some a → enumerate sel s (k + m) = some a → k + m = k
s : Set α
h₁ : enumerate sel s (Nat.succ k) = some a
h₂ : enumerate sel s (Nat.succ k + m) = some a
⊢ Nat.succ k + m = Nat.succ k
cases h : sel s
case some
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
m k : ℕ
ih : ∀ {s : Set α}, enumerate sel s k = some a → enumerate sel s (k + m) = some a → k + m = k
s : Set α
h₁ : enumerate sel s (Nat.succ k) = some a
h₂ : enumerate sel s (Nat.succ k + m) = some a
val✝ : α
h : sel s = some val✝
⊢ Nat.succ k + m = Nat.succ k
case none
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
m k : ℕ
ih : ∀ {s : Set α}, enumerate sel s k = some a → enumerate sel s (k + m) = some a → k + m = k
s : Set α
h₁ : enumerate sel s (Nat.succ k) = some a
h₂ : enumerate sel s (Nat.succ k + m) = some a
h : sel s = none
⊢ Nat.succ k + m = Nat.succ k

case some
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
m k : ℕ
ih : ∀ {s : Set α}, enumerate sel s k = some a → enumerate sel s (k + m) = some a → k + m = k
s : Set α
h₁ : enumerate sel s (Nat.succ k) = some a
h₂ : enumerate sel s (Nat.succ k + m) = some a
val✝ : α
h : sel s = some val✝
⊢ Nat.succ k + m = Nat.succ k
case none =>
  simp_all only [add_comm, self_eq_add_left, Nat.add_succ, enumerate_eq_none_of_sel _ h]
no goals
case some
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
m k : ℕ
ih : ∀ {s : Set α}, enumerate sel s k = some a → enumerate sel s (k + m) = some a → k + m = k
s : Set α
h₁ : enumerate sel s (Nat.succ k) = some a
h₂ : enumerate sel s (Nat.succ k + m) = some a
val✝ : α
h : sel s = some val✝
⊢ Nat.succ k + m = Nat.succ k
case some _ =>
  simp_all only [add_comm, self_eq_add_left, enumerate, Option.some.injEq,
                 Nat.add_succ, enumerate._eq_2, Nat.succ.injEq]
  exact ih h₁ h₂
no goals
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
m k : ℕ
ih : ∀ {s : Set α}, enumerate sel s k = some a → enumerate sel s (k + m) = some a → k + m = k
s : Set α
h₁ : enumerate sel s (Nat.succ k) = some a
h₂ : enumerate sel s (Nat.succ k + m) = some a
h : sel s = none
⊢ Nat.succ k + m = Nat.succ k
simp_all only [add_comm, self_eq_add_left, Nat.add_succ, enumerate_eq_none_of_sel _ h]
α : Type u_1
sel : Set α → Option α
a : α
m k : ℕ
s : Set α
val✝ : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
ih : ∀ {s : Set α}, enumerate sel s k = some a → enumerate sel s (m + k) = some a → m + k = k
h₁ :
  (do
      let a ← some val✝
      enumerate sel (s \ {a}) k) =
    some a
h₂ :
  (do
      let a ← some val✝
      enumerate sel (s \ {a}) (m + k)) =
    some a
h : sel s = some val✝
⊢ m + k = k
α : Type u_1
sel : Set α → Option α
a : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
m k : ℕ
ih : ∀ {s : Set α}, enumerate sel s k = some a → enumerate sel s (k + m) = some a → k + m = k
s : Set α
h₁ : enumerate sel s (Nat.succ k) = some a
h₂ : enumerate sel s (Nat.succ k + m) = some a
val✝ : α
h : sel s = some val✝
⊢ Nat.succ k + m = Nat.succ k
simp_all only [add_comm, self_eq_add_left, enumerate, Option.some.injEq,
               Nat.add_succ, enumerate._eq_2, Nat.succ.injEq]
no goals
α : Type u_1
sel : Set α → Option α
a : α
m k : ℕ
s : Set α
val✝ : α
h_sel : ∀ (s : Set α) (a : α), sel s = some a → a ∈ s
ih : ∀ {s : Set α}, enumerate sel s k = some a → enumerate sel s (m + k) = some a → m + k = k
h₁ :
  (do
      let a ← some val✝
      enumerate sel (s \ {a}) k) =
    some a
h₂ :
  (do
      let a ← some val✝
      enumerate sel (s \ {a}) (m + k)) =
    some a
h : sel s = some val✝
⊢ m + k = k
exact ih h₁ h₂