local function next_number (last, current)
	current = current + 1
	if current > last then
		return nil
	end
	return current
end


local function range (first, last)
	if first > last then
		error "start of range must be less or equal to last value"
	end
	return next_number, last, first - 1
end


local generate = {
	next_number = next_number,
	range = range,
}


function generate.trange (first, last)
	local x = {}
	for i in range(first, last) do
		table.insert(x, i)
	end
	return x
end


return generate