# Print the tree starting from `node`.
	protected fun print_tree(node: N): String do
		var s = new FlatBuffer
		s.append(node.to_s)
		if node.left != null then s.append(print_tree(node.left.as(not null)))
		if node.right != null then s.append(print_tree(node.right.as(not null)))
		return s.to_s
	end