∀ {α : Type u_1} {β : α → Type u_2} (l l'' : Std.DTreeMap.Internal.Impl α β)
  (motive : Std.DTreeMap.Internal.Impl.Tree α β (l.size + l''.size) → Sort u_3) (ℓ : Std.DTreeMap.Internal.Impl α β)
  (hℓ₁ : ℓ.Balanced) (hℓ₂ : ℓ.size = l.size + l''.size)
  (h_1 :
    (ℓ : Std.DTreeMap.Internal.Impl α β) →
      (hℓ₁ : ℓ.Balanced) →
        (hℓ₂ : ℓ.size = l.size + l''.size) → motive { impl := ℓ, balanced_impl := hℓ₁, size_impl := hℓ₂ }),
  (match { impl := ℓ, balanced_impl := hℓ₁, size_impl := hℓ₂ } with
    | { impl := ℓ, balanced_impl := hℓ₁, size_impl := hℓ₂ } => h_1 ℓ hℓ₁ hℓ₂) =
    h_1 ℓ hℓ₁ hℓ₂