non virtual interfaces

[Alexandr Druzhinin]

if I use protected instead of private in interface like:
interface Transmogrifier
{
	final void thereAndBack()
	{
		transmogrify();
		untransmogrify();
	}

	protected:
		void transmogrify();
		void untransmogrify();
}

class CardboardBox: Transmogrifier
{
	override protected void transmogrify() { }
	override void untransmogrify() {}
}

int main()
{
	auto cb = new CardboardBox();
	return 0;
}

it compiles, but why does compiler permit making untransmogrify() be 
public? How can I prohibit this? May be it just unrealized yet?