∀ {α : Type u} {β : α → Type v} {k : α} {v : β k} {l r : Std.DTreeMap.Internal.Impl α β},
  l.Balanced →
    r.Balanced →
      Std.DTreeMap.Internal.Impl.BalanceLErasePrecond r.size l.size →
        Std.DTreeMap.Internal.Impl.balanceR! k v l r = Std.DTreeMap.Internal.Impl.balance! k v l r