protected Node<E> addLeaf (E e, Node<E> leftLeaf, Node<E> rightLeaf) {
		assert e != null : "e != null";
		assert (leftLeaf == null || leftLeaf.isLeaf) 
		&& (rightLeaf == null || rightLeaf.isLeaf) :
			"leftLeaf and rightLeaf must be leafs";
		
		Node<E> n = new Node<E>(e);//for TTree, ignore left and right neighbors of this new leaf
		size++;
		return n;
	}