test_to_key = function()
	check_eq(to_key(0), 'a', 0)
	check_eq(to_key(1), 'b', 1)
	-- ...
	check_eq(to_key(24), 'y', 24)
	check_eq(to_key(25), 'z', 25)
	check_eq(to_key(26), 'ba', 26)
	check_eq(to_key(26*26), 'baa', '3 digit')
	check_eq(to_key(26*26+1), 'bab', '3 digit/2')
	check_eq(to_key(26*26*26), 'baaa', '4 digit')
end

to_key = function(n)
	-- represent an integer n in base-26 using a-z
	local result = {}
	n = math.floor(n)
	local a = string.byte('a')
	while n > 0 do
		local digit = n % 26
		table.insert(result, 1, string.char(digit+a))
		n = math.floor(n/26)
	end
	if #result == 0 then
		return 'a'
	end
	return table.concat(result)
end

next_key = function()
	local result = to_key(First_available_id)
	First_available_id = First_available_id+1
	return result
end

First_available_id = 1

	for _,node in ipairs(Nodes) do

	for _,node in pairs(Nodes) do

end

end

	for _,node in ipairs(Nodes) do

	for _,node in pairs(Nodes) do

end

end

	{type='text', x=0,y=0, margin=0, width=120},

	a={type='text', x=0,y=0, margin=0, width=120},

	for i,node in ipairs(Nodes) do
		node.id = i

	for key,node in pairs(Nodes) do
		node.id = key