# Push down the `node` in tree from a specified `from` index
	protected fun shift_down(from, node: N) do
		var cmp = node.key <=> from.key
		if cmp < 0 then
			if from.left == null then
				from.left = node
				node.parent = from
			else
				shift_down(from.left.as(not null), node)
			end
		else if cmp > 0 then
			if from.right == null then
				from.right = node
				node.parent = from
			else
				shift_down(from.right.as(not null), node)
			end
		end
	end