# Add `node` in the tree.
	protected fun insert_node(node: N) do
		len += 1
		if root == null then
			root = node
		else
			shift_down(root.as(not null), node)
		end
		if first_node == null then
			first_node = node
		end
		if last_node != null then
			last_node.next = node
			node.prev = last_node
		end
		last_node = node
	end