∀ {α : Type u_1} {β : α → Type u_2} (sz : ℕ) (k' : α) (v' : β k') (l' r' : Std.DTreeMap.Internal.Impl α β)
  (hl : (Std.DTreeMap.Internal.Impl.inner sz k' v' l' r').Balanced) (r'' : Std.DTreeMap.Internal.Impl α β)
  (hr''₁ : r''.Balanced) (hr''₂ : r''.size = r'.size + 1),
  (Std.DTreeMap.Internal.Impl.balanceR k' v' l' r'' ⋯ ⋯ ⋯).size =
    (Std.DTreeMap.Internal.Impl.inner sz k' v' l' r').size + 1