∀ {α : Type u_1} {β : α → Type u_2} {sz : ℕ} {k : α} {v : β k} {l r : Std.DTreeMap.Internal.Impl α β}
  (motive : (Std.DTreeMap.Internal.Impl.inner sz k v l r).Balanced → Prop)
  (x : (Std.DTreeMap.Internal.Impl.inner sz k v l r).Balanced),
  (∀ (a : l.Balanced) (h : r.Balanced) (a_1 : Std.DTreeMap.Internal.Impl.BalancedAtRoot l.size r.size)
      (a_2 : sz = l.size + 1 + r.size), motive ⋯) →
    motive x